
Class jJp^M^ 
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Total Eclipse of the Sun of May 29, 1900. 
Photographed by the party of the Smithsonian Institution. 



Science for Everybody 



ASTRONOMY FOR 
EVERYBODY 

A Popular Exposition of the Wonders 
of the Heavens 

BY 

SIMON NEWCOMB, LL.D. 

Professor^ U. S. iV., retired 



FULLY ILLUSTRATED 



NEW YORK 

McCLURE, PHILLIPS & CO. 

1902 





» 


THE E-'-YCF 


T^vo ' -■-- '--■'^•..'i 




NOV, 10 t902 




CLASS CK., XaC. V:0 




Copyright, 1902, by 


URE, PHILLIPS 


& CO. 



Puhlishffd.JSi^yamiher, 1902, N 



Preface 



The present work grew out of articles contributed to 
McClure's Magazine a few years since on the Unsolved 
Problems of Astronomy, Total Eclipses of the Sun, and 
other subjects. The interest shown in these articles 
suggested an exposition of the main facts of astronomy 
in the same style. The result of the attempt is now 
submitted to the courteous consideration of the reader. 

The writer who attempts to set forth the facts of as- 
tronomj" without any use of technical language finds him- 
self in the dilemma of being obliged either to convey 
only a very imperfect idea of the subject, or to enter 
upon explanations of force and motion which his reader 
ma3^ find tedious. In grappling with this difficulty the 
author has followed a middle course, trying to present 
the subject in such a way as to be intelligible and inter- 
esting to every reader, and entering into technical ex- 
planations onh'- when necessary to the clear understand- 
ing of such matters as the measure of time, the changes 
of the seasons, the varying positions of the constella- 
tions, and the aspects of the planets. It is hoped that 
the reader who does not wish to master these subjects 
will find enough to iaterest him in the descriptions and 
illustrations of celestial scenery to which the bulk of the 
Avork is devoted. 

The author is indebted to Mr. Secretary Langley, of 
the Smithsonian Institution, for the use of the picture 
which forms the frontispiece. 

Simon Newcomb. 

Washington, October, 1902. 



Contents 



PART I. THE CELESTIAL MOTION'S 

PAGE 

I. A View of the Universe 3 

What tlie universe is 6 

A model of the universe = 7 

n. Aspects of the Heavens 9 

Apparent daily revolution of the stars o 12 

Changes in the motions as we journey south 15 

rH. Eelation of Time and Longitude 19 

Local time 21 

Standard time 21 

Where the day changes o 23 

rV. How the Position of a Heavenly Body is Defined ^5 

Circles of the celestial sphere 27 

V. The Annttal Motion of the Earth and its Eesults « . . 31 

The sun's apparent path in the sky 32 

The ecliptic 33 

The equinoxes and solstices 37 

The seasons 39 

Eelations between real and apparent motions summed 

up 39 

The year and the precession of the equinoxes 41 

PART IL ASTRONOMICAL INSTRUMENTS 

I. The Eefracting Telescope 47 

The lenses of a telescope 48 

The image of a distant object 51 

Power and defects of a telescope 53 

Mounting of the telescope 55 

The making of telescopes 59 

Fraunhof er and Alvan Clark 61 



vili ■ CONTENTS 

PAttK 

n. The Eeflecting Telescope 67 

in. The Photographic Telescope „ 71 

IV. The Spectroscope 73 

Nature and wave length of light 74 

The spectrum 75 

How the stars are analysed • 76 

V. Other Astronomical Instruments 79 

The meridian circle and clock 80 

How the position of stars are determined. 81 

PART III. THE SUN, EARTH, AND MOON 

I. An Introductory Glance at the Solar System « . 87 

II. The Sun 91 

General description 91 

Rotation of the sun 93 

Density and gravity 94 

Spots on the sun 95 

TheFaculae 98 

The prominences and chromosphere 99 

How the sun is made up 101 

The source of the sun's heat. 103 

III. The Earth 107 

Measuring the earth 107 

The earth's interior 108 

Its gravity and density Ill 

Variations of latitude 114 

The atmosphere 116 

IV. The Moon 119 

Distance of the moon 120 

Revolution and phases 120 

The surface of the moon 123 

Is there air or water on the moon ? 127 

Rotation of the moon 128 

How the moon produces the tides 129 

V. Eclipses of the Moon 133 

The nodes of the moon's orbit 134 

Eclipse seasons » . . 134 

How an eclipse of the moon looks 136 



CONTENTS ix 

PAGE 

VI. Eclipses of the Sun ^ 139 

Central, total, and annular eclipses 140 

Beanty of a total eclipse 141 

Ancient eclipses 143 

Prediction of eclipses 144 

The sun's appendages » 145 

The corona 147 

PART IV. THE PLANETS AND THEIR SATELLITES 

I. Orbits and Aspects of the Planets 151 

Distances of the planets 154 

Bode's law 154 

Kepler's laws 155 

II. The Planet Mercury 157 

Surface and rotation of Mercury 159 

Observations of Schroter, Herschel, Schiaparelli, and 

Lowell , 160 

Phases of Mercury 161 

Transits of Mercury . . „ 162 

m. The Planet Venus » 167 

The morning and evening star 167 

Eotation of Venus 169 

Atmosphere of Venus 172 

Has Venus a satellite ? 174 

Transits of Venus . , 175 

IV. The Planet Mars 177 

Distance, dates of opposition, etc 178 

Surface and rotation of Mars 179 

The canals of Mars 180 

Probable nature of the channels 184 

The atmosphere of Mars 185 

Supposed winter snowfall near the poles 187 

The satellites of Mars 188 

Y. The Group of Minor Planets 191 

Discovery of Ceres 191 

Hunting asteroids , 192 

Orbits of the asteroids 194 

Groupings of the orbits 195 

The most curious of the asteroids 198 



X CONTENTS 

PAGE 

VI. Jupiter and its Satellites 201 

Aspect of Jupiter 201 

Surface 202 

Constitution 205 

Eotation 206 

Eesemblance of Jupiter to the sun : 206 

The satellites of Jupiter 208 

VII. Saturn and its System 213 

Aspects of Saturn , 213 

Satellites of Saturn 214 

Varying aspects of Saturn's rings 215 

What the rings are 219 

System of Saturn's satellites 220 

Physical constitution of Saturn 224 

VIII. Uranus and its Satellites 225 

Discovery of Uranus 225 

Old observations 226 

Constitution of the planet 227 

Its satellites 227 

IX. Neptune and its Satellite 231 

History of the discovery of Neptune 282 

Satellite of Neptune 235 

X. How the Heavens are Measured 237 

Parallax 237 

Measurement by the motion of light 239 

Measurement by the sun's gravitation 240 

Eesults of measurements of the sun's distance 242 

XI. Gravitation and the Weighing of the Planets 243 

Accuracy of astronomical predictions based on the 

theory of gravitation 244 

How the planets are weighed , . . 246 

PART V. COMETS AND METEOHIG BODIES 

I. Comets 255 

Description of a comet 255 

Orbits of comets ; 257 

Halley's comet 260 

Comets which have disappeared 262 



CONTENTS xi 

PAGE 

Encke's comet 264 

Capture of comets by Jupiter o . . . 265 

Wlience come comets ? 266 

Brilliant comets of our time 267 

Nature of comets 274 

II. Meteoric Bodies 277 

Meteors 277 

Cause of meteors 278 

Meteoric showers 279 

Connection of comets and meteors , . . . , 281 

The zodiacal light 283 

The impulsion of light 286 

PART YI. THB FIXED STAMS 

I. General Eeview '. 291 

Stars and nebulae :. 293 

Spectra of the stars , 293 

Density and heat of the stars 296 

II. Aspect of the Sky 299 

The Milky Way.'. ". 299 

---- Brightness of the stars , 300 

Number of stars 301 

'Colours 303 

Collection into constellations 303 

III. Description of the Constellations 305 

To find the sidereal time 306 

The northern constellations 307 

The autumnal constellations , 309 

The winter constellations , ; . -. ... 313 

The spring constellations ^ 316 

The summer constellations -. ; . . ' 317 

TV. The Distances of the Stars ,. . . 321 

V. The Motions op the Stars. , . . ." 325 

VI. Variable and Compound Stars 329 



List of Illustrations 



PAGE 

Total Eclipse of the Sun of May 29, 1900. Photographed by the 

party of the Smithsonian Institution Frontispiece 

The Celestial Sphere as it appears to us 13 

The Northern Sky and the Pole Star 16 

Circles of the Celestial Sphere 27 

The Sun Crossing the Equator about March Twentieth 33 

The Orbit of the Earth and the Zodiac 33 

How the Obliquity of the Ecliptic Produces the Changes of Seasons 35 
Apparent Motion of the Sun along the Ecliptic in Spring and 

Summer 36 

Apparent Motion of the Sun from March till September 37 

Precession of the Equinoxes 43 

Section of the Object-glass of a Telescope 50 

Axes on which a Telescope turns , ....... 57 

Great Telescope of the Yerkes Observatory 65 

Section of a Newtonian Eeflecting Telescope 69 

Wave Length of Light 74 

Arrangement of the Colours of the Spectrum 75 

A Meridian Instrument 80 

Appearance of a Sun-spot 96 

Frequency of Sun-spots in Different Latitudes on the Sun ...... 97 

Eevolution of the Moon Eound the Earth , 121 

Mountainous Surface of the Moon , 124 

Showing how the Moon would Move if it did not Eotate on its 

Axis 129 

How the Moon Produces Two Tides in a Day 131 

The Moon in the Shadow of the Earth 133 



xiv LIST OF ILLUSTRATIONS 

PAGE 

Passage of the Moon through the Earth's Shadow 136 

The Shadow of the Moon Thrown on the Earth during a Total 

Eclipse of the Sun 139 

The Moon Passing Centrally over the Sun during an Annular 

Eclipse 140 

Orbits of the Four Inner Planets ■ 152 

Conjunctions of Mercury with the Sun 158 

Elongations of Mercury 159 

Phases of Venus in Different Points of its Orbit 168 

Effect of the Atmosphere of Venus during the Transit of 1882. . 172 
Map of Mars and its Canals as drawn at the Lowell Observatory 181 
Drawings of Lacus Solis on Mars, by Messrs. Campbell and 

Hussey 188 

Separation of the Minor Planets into Groups 195 

Distribution of the Orbits of the Minor Planets 196 

Telescopic Views of Jupiter, one with the Shadow of a Satellite 

Crossing the Planet 204 

Perpendicular View of the Eings of Saturn. 216 

Showing how the Direction of the Plane of Saturn's Rings re- 
mains Unchanged 217 

Disa^jpearance of the Rings of Saturn, according to Barnard, 

when seen edgewise 218 

Orbits of Titan and Hyperion, showing their relation , . . . 222 

Measure of the Distance of an Inaccessible Object by Triangula- 

tion 237 

Parabolic Orbit of a Comet 257 

Donati's Comet, as drawn by G. P. Bond 268 

Head of Donati's Comet, drawn by G. P. Bond 270 

Great Comet of 1859, drawn by G. P. Bond 272 

The Zodiacal Light in February and March 284 

Ursa Major, or The Dipper 307 

Ursa Minor 308 

Cassiopeia 308 

J^yra, the Harp 311 



LIST OF ILLUSTRATIONS xv 

PAGE 

The Hyades 313 

The Pleiades, as seen with the naked eye 313 

Telescopic View of the Pleiades, with Names of the Brighter Stars 314 

Orion 316 

The Northern Crown 317 

Aquila 318 

Delphinns, the Dolphin 318 

The Great Cluster of Hercules, photographed at the Lick Observ- 
atory 319 

Scorpius, the Scorpion 320 

Measurement of the Parallax of a Star , 322 

Arcturus and the Surrounding Stars in Constellation Bootes 328 



PART I 



THE CELESTIAL MOTIONS 



A View of the Universe 

Let us enter upon our subject by taking a general 
view of this universe in which we live, fancying ourselves 
looking at it from a point without its limits. Far away, 
indeed, is the point we must choose. To give a concep- 
tion of the distance, let us measure it by the motion of 
light. This agent, darting through 186,000 miles in 
every second, would make the circuit of the earth several 
times between two ticks of a watch. The standpoint 
which we choose will probably be well situated if we take 
it at a distance through which light would travel in 
100,000 years. So far as we know, we should at this 
point find ourselves in utter darkness, a black and star- 
less sky surrounding us on all sides. But, in one direc- 
tion, we should see a large patch of feeble light spread- 
ing over a considerable part of the heavens like a faint 
cloud or the first glimmer of a dawn. Possibly there 
might be other such patches in different directions, but 
of these we know nothing. The one which we have men- 
tioned, and which we call the universe, is that which we 
are to inspect. We therefore fly toward it — how fast we 
need not say. To reach it in a month we should have 
to go a million times as fast as light. As we approach, 
it continually spreads out over more of the black sky, 



4 THE CELESTIAL MOTIONS 

which it at length half covers, the region behind us being 
still entirely black. 

Before reaching this stage we begin to see points of 
light glimmering here and there in the mass. Continu- 
ing our course, these points become more numerous, and 
seem to move past us and disappear behind us in the 
distance, wliile new ones continually come into view in 
front, as the passengers on a railway train see landscape 
and houses flit by them. These are stars, wliich, when 
we get well in among them, stud the whole heavens as 
we see them do at night. We might pass through the 
whole cloud at the enormous speed we have fancied, with- 
out seeing anything but stars and, perhaps, a few great 
nebulous masses of foggy light scattered here and there 
among them. 

But instead of doing this, let us select one particular 
star and slacken our speed to make a closer inspection 
of it. This one is rather a small star; but as we ap- 
proach it, it seems to our eyes to grow brighter. In time 
it shines like Venus. Then it casts a shadow; then we 
can read by its light; then it begins to dazzle our eyes. 
It looks like a little sun. It is the Sun ! 

Let us get into a position which, compared with the dis- 
tances we have been travelling, is right alongside of the 
sun, though, expressed in our ordinary measure, it may 
be a thousand million miles away. Now, looking down 
and around us, we see eight star-like points scattered 
around the sun at different distances. If we watch them 
long enough we shall see them all in motion around the 
sun, completing their circuit in times ranging from three 



A MEW OF THE UNIVERSE 5 

months to more than 160 3'cars. They move at very 
different distances; tlie most distant is seventy times as 
far as tlie nearest. 

These star-hke bodies are the planets. By careful 
examination we see that they differ from the stars in 
being opaque bodies, shining only by light borrowed 
from the sun. 

Let us pay one of them a visit. We select the third 
in order from the sun. Approaching it in a direction 
which we may call from above, that is to say from a 
direction at right angles to the line drawn from it to 
the sun, we see it grow larger and brighter as we get 
nearer. When we get very near, we see it looking like 
a half-moon — one hemisphere being in darkness and the 
other illuminated by the sun's rays. As we approach 
3"et nearer, the illuminated part, always growing larger 
to our sight, assumes a mottled appearance. Still ex- 
panding, this appearance gradually resolves itself into 
oceans and continents, obscured over perhaps half their 
surface by clouds. The surface upon which we are look- 
ing continually spreads out before us, filling more and 
more of the sky, until we see it to be a world. We land 
upon it, and here we are upon the earth. 

Thus, a point which was absolutely invisible while we 
were flying through the celestial spaces, which became a 
star when we got near the sun, and an opaque globe when 
yet nearer, now becomes the world on which we live. 

This imaginary flight makes known to us a capital 
fact of astronomy: The great mass of stars which stud 
the heavens at night are suns. To express the idea in 



6 THE CELESTIAL MOTIONS 

another way, the sun is merely one of the stars. Com- 
pared with its fellows it is rather a small one, for we 
know of stars that emit thousands or even tens of thou- 
sands of times the light and heat of the sun. Measur- 
ing things simply by their intrinsic importance, there is 
nothing special to distinguish our sun from the hundreds 
of millions of its companions. Its importance to us and 
its comparative greatness in our eyes arise simply from 
the accident of our relation to it. 

The great universe of stars which we have described 
looks to us from the earth just as it looked to us during 
our imaginary flight through it. The stars which stud 
our sky are the same stars which we saw on our flight. 
The great difference between our view of the heavens 
and the view from a point in the starry distances is the 
prominent position occupied by the sun and planets. 
The former is so bright that during the daytime it com- 
pletely obliterates the stars. If we could cut off* the 
sun's rays from any very wide region, we should see the 
stars around the sun in the daytime as well as by night. 
These bodies surround us in all directions as if the earth 
were placed in the centre of the universe, as was sup- 
posed by the ancients. 

What the Universe Is 

We may connect what we have just learned about the 
the universe at large with what we see in the heavens. 
What we call the heavenly bodies are of two classes. 
One of these comprises the millions of stars the arrange- 
ment and appearance of which we have just described. 



WHAT THE UNIVERSE IS 7 

The other comprises a single star, which is for us the 
most important of all, and the bodies connected with it. 
This collection of bodies, with the sun in its centre, forms 
a little colony all by itself, which we call the solar system. 
The feature of this system which I wish first to impress 
on the reader's mind is its very small dimensions when 
compared with the distances between the stars. All 
around it are spaces which, so far as we yet know, are 
quite void through enormous distances. If we could fly 
across the whole breadth of the system, we should not be 
able to see that we were any nearer the stars in front of 
us, nor would the constellations look in any way different 
from what they do from our earth. An astronomer armed 
with the finest instruments would be able to detect a 
change only by the most exact observations, and then 
only in the case of the nearer stars. 

A conception of the respective magnitudes and dis- 
tances of the heavenly bodies, which will help the reader 
in conceiving of the universe as it is, may be gained by 
supposing us to look at a little model of it. Let us 
imagine that, in this model of the universe, the earth on 
which we dwell is represented by a grain of mustard seed. 
The moon will then be a particle about one fourth the 
diameter of the grain, placed at a distance of an inch 
from the earth. The sun will be represented by a large 
apple, placed at a distance of forty feet. Other planets, 
ranging in size from an invisible particle to a pea, must 
be imagined at distances from the sun varying from ten 
feet to a quarter of a mile. We must then imagine all 
these little objects to be slowly moving around the 



8 THE CELESTIAL MOTIONS 

sun at their respective distances, in times varying from 
three months to 160 years. As the mustard seed per- 
forms its revolution in the course of a year we must 
imagine the moon to accompany it, making a revolution 
around it every month. 

On this scale a plan of the whole solar system can be 
laid down in a field half a mile square. Outside of this 
field we should find a tract broader than the whole con- 
tinent of America without a visible object in it unless 
perhaps comets scattered around its border. Far beyond 
the limits of the American continent we should find the 
nearest star, which, hke our sun, might be represented 
by a large apple. At still greater distances, in every 
direction, would be other stars, but, in the general aver- 
age, they would be separated from each other as widely 
as the nearest star is from the sun. A region of the 
little model as large as the whole earth might contain 
only two or three stars. 

We see from this how, in a flight through the universe, 
like the one we' have imagined, we might overlook such 
an insignificant little body as our earth, even if we made 
a careful search for it. We should be like a person fly- 
ing through the Mississippi Valley, looking for a grain 
of mustard seed wliich he knew was hidden somewhere 
on the American continent. Even the bright shining 
apple representing the sun might be overlooked unless 
we happened to pass quite near it. 



II 

Aspects of the Heavens 

The immensity of the distances wLich separate us 
from the heavenly bodies makes it impossible for us to 
form a distinct conception of the true scale of the uni- 
verse, and very difficult to conceive of the heavenly 
bodies in their actual relations to us. If, on looking at a 
body in the sky, there were any way of estimating its 
distance, and if our eyes were so keen that we could see 
the minutest features on the surface of the planets and 
stars, the true structure of the universe would have been 
obvious from the time that men began to study the heav- 
ens. A little reflection will make it obvious that if we 
could mount above the earth to a distance of, say. ten 
thousand times its diameter, so that it would no longer 
have any perceptible size, it would look to us, in the light 
of the sun, like a star in the sky. The ancients had no 
conception of distances like this, and so supposed that 
the heavenly bodies werej> as they appeared, of a con- 
stitution totally different from that of the earth. We 
ourselves, looking at the heavens, are unable to conceive 
of the stars being millions of times farther than the 
planets. All look as if spread out on one sky at the same 
distance. We have to learn their actual arrangement 
and distances by reason. 

It is from the impossibility of conceiving these enor- 



10 THE CELESTIAL MOTIONS 

mous differences in the distances of objects on the earth 
and the heavens, that the real difficulty of forming a 
mental picture of them in their true relation arises. I 
shall ask the reader's careful attention in an attempt 
to present these relations in the simplest way, so as to 
connect things as they are with things as we see them. 

I^et us suppose the earth taken away from under our 
feet, leaving us hanging in mid space. We should then 
see the heavenly bodies — sun, moon, planets, and stars — 
surrounding us in every direction, up and down, east and 
w^est, north and south. The eye would rest on nothing 
else. As we have just explained, all these objects would 
seem to us to be at the same distance. 

A great collection of points scattered in every direc- 
tion at an equal distance from one central point, must 
all lie upon the inner surface of a hollow^ sphere. It fol- 
lows that, in the case supposed, the heavenly bodies will 
appear to us as if set in a sphere in the centre of which 
we appear to be placed. Since one of the final objects 
of astronom}" is to learn the directions of the heavenly 
bodies from us, this apparent sphere is talked about in 
astronomy as if it were a reality. It is called the celes- 
tial sphere. In the case we have supposed, with the earth 
out of the way, all the heavenly bodies on this sphere 
would at any moment seem at rest. The stars would re- 
main apparently at rest day after day and week after 
w^eek. It is true that, by watching the planets, we should 
in a few days or weeks, as the case might be, see their 
slow motion around the sun, but this would not be per- 
ceptible at once. Our first impression would be that the 



ASPECTS OF THE HEAVENS 11 

sphere was made of some solid, crystalline substance, 
and that the heavenly bodies were fastened to its inner 
surface. The ancients had this notion, which they 
brought yet nearer the truth by fancying a number of 
these spheres fitting inside of each other to represent the 
different distances of the heavenly bodies. 

With this conception well in mind, let us bring the 
earth back under our feet. Now we have to make a draft 
upon the reader's power of conception. Considered in 
its relation to the magnitude of the heavens, the earth 
is a mere point ; 3^et, when we bring it into place, its sur- 
face cuts off one half of the universe from our view, j ust 
as an apple would cut off the view of one side of a room 
from an insect crawling upon it. That half of the celes- 
tial sphere which, being above the horizon, remains visi- 
ble is called the visible hemisphere; the half below, the 
view of which is cut off by the earth, is called the invisi- 
ble hemisphere. Of course we could see the latter by 
travelling around the earth. 

Having this state of things well in mind, we must 
make another draft on the reader's attention. We know 
that the earth is not at rest, but revolves unceasingly 
around an axis passing through its centre. The natural 
result of this is an apparent rotation of the celestial 
sphere in the opposite direction. The earth rotates from 
west toward east ; hence the sphere seems to rotate from 
east toward west. This real revolution of the earth, with 
the apparent revolution of the stars which it causes, is 
called the diurnal motion, because it is completed in a 
day. 



12 THE CELESTIAL MOTIONS 

Appaj'ent Daily BcroJuiion of the Stars 

Our next problem is to sliow the connection between the 
very simple conception of the rotation of the earth and 
the more complicated appearance presented by the ap- 
parent diurnal motion of the heavenly bodies which it 
brings about. The latter varies with the latitude of the 
observer upon the earth's surface. Let us begin with its 
appearance in our middle northern latitudes. 

For this purpose we may in imagination build a hollow 
globe representing the celestial sphere. We may make 
it as large as a Ferris wheel, but one of thirty or forty 
feet in diameter would answer our purpose. Let Figure 
1 be an inside view of this globe, mounted on two pivots, 
P and Q, so that it can turn round on them diagonally. 
In the middle, at O, we have a horizontal platform, NS, 
on which we sit. The constellations are marked on the 
inside of the globe, covering the whole surface, but 
those on the lower half are hidden from view by the 
platform. This platform, as is evident, represents the 
horizon. 

The globe is now made to turn on its pivots. What 
will happen? We shall see the stars near the pivot P 
revolving around the latter as the globe turns. The 
stars on a certain circle KN will graze the edges of the 
platform, as they pass below P. Those jet farther from 
P will dip below the platform to a greater or less extent, 
according to their distance from P. Stars near the circle 
EF, halfway between P and Q, will perform half their 
course above, and half below the platform. Finally, 



REVOLUTION OF THE STARS 



IS 



stars within the circle ST will never rise ahove the level 
of the platform at all, and will remain invisible to us. 

To our eves the celestial sphere is such a globe as this, 
of infinite dimensions. It seems to us to be continually 




Fig. 1. — The Celestial Sphere as it appears to us. 

revolving round a certain point in the sky as a pivot, 
making one revolution in nearl}^ a day, and carrying the 
sun, moon, and stars wdth it. The stars preserve their 
relative positions as if fastened to the revolving celestial 
sphere. That is to say, if we take a photograph of them 



14 THE CELESTIAL MOTIONS 

at any hour of the night, the same photograph will show 
their appearance at any other hour, if we only hold it in 
the right position. 

The pivot corresponding to P is called the north celes- 
tial pole. To dwellers in middle northern latitudes, where 
most of us live, it is in the northern sky, nearly midway 
between the zenith and the northern horizon. The 
farther south we live, the nearer it is to the horizon, its 
altitude above the latter being equal to the latitude of 
the place where the observer stands. Quite near it is the 
pole star, which we shall hereafter show how to locate. 
To ordinary observation, the pole star seems never to 
move from its position. In our time it is little more than 
a degree from the pole, a quantity with which we need 
not now concern ourselves. 

Opposite the north celestial pole, and therefore as far 
below our horizon as the north one is above it, lies the 
south celestial pole. 

An obvious fact is that the diurnal motion as we see 
it in our latitude is oblique. When the sun rises in the 
east it does not seem to go straight up from the horizon, 
but moves over toward the south at a more or less acute 
angle with the horizon. So when it sets, its motion rela- 
tive to the horizon is again oblique. 

Now, imagine that we take a pair of compasses long 
enough to reach the sky. We put one point on the sky 
at the north celestial pole, and the other point far 
enough from it to touch the horizon below the pole. 
Keeping the first point at the pole we draw a complete 
circle on the celestial sphere with the other point. This 



REVOLUTION OF THE STARS 15 

circle just touches the north horizon at its lowest point 
and, in our northern latitudes, extends to near the zenith 
at its highest point. The stars within this circle never 
set, but only seem to perform a daily course around the 
pole. For this reason this circle is called the circle of 
perpetual apparition. 

The stars farther south rise and set, but perform less 
and less of their daily course above our horizon, till 
we reach the south point of it, where they barely show 
themselves. 

Stars yet farther south never rise at all in our lati- 
tudes. They are contained within the circle of perpetual 
occultation, which surrounds and is centred on the south 
celestial pole, as the circle of perpetual apparition is 
centred on the north one. 

Figure S shows the principal stars of the northern 
heavens within the circle of perpetual apparition for the 
Northern States. By holding it with the month on top 
we shall have a view of the constellations as they are seen 
about eight o'clock in the evening. It also shows how to 
find the pole star in the centre by the direction of the 
two outer stars or pointers in the Dipper, or Great Bear. 

Now let us change our latitude and see what occurs. 
If we journey toward the equator, the direction of our 
horizon changes, and during our voyage we see the pole 
star constantly sinking lower and lower. As we ap- 
proach the equator, it approaches the horizon, reaching 
it when we reach the equator. It is plain enough that 
the circle of perpetual apparition grows smaller until, 
at the equator, it ceases to exist, each pole being in our 



16 



THE CELESTIAL MOTIONS 



horizon. Now the diurnal motion seems to us quite dif- 
ferent from what it is here. The sun, moon, and stars, 
when they rise, commence their motion directly upwards. 
If one of them rises exactly in the east, it will pass 




/IX 

Fig. 2.— The Xorthcrn Sky and the Pole Star. 



through the zenith ; one rising south of the east will pass 
south of the zenith; one rising north of the east, north 
of the zenith. 

Continuing our course into the southern hemisphere, 
we find that the sun, while still rising in the east, gener- 
ally passes the meridian to the north of the zenith. The 



REVOLUTION OF THE STARS 17 

main point of difference between the two hemispheres is 
that, as the sun now culminates in the north, its ap- 
parent motion is not in the direction of the hands of a 
watch, as with us, but in the opposite direction. In 
middle southern latitudes, the northern constellations, 
so familiar to us, are always below the horizon, but we 
see new ones in the south. Some of these are noted for 
their beauty, the Southern Cross, for example. Indeed, 
it has often been thought that the southern heavens 
were more brilliant and contained more stars than the 
northern ones. But this view is now found to be incor- 
rect. Careful study and counts of the stars show the 
number to be about the same in one hemisphere as in the 
other. Probably the impression we have mentioned 
arose from the superior clearness of the sky in the 
southern regions. For some reason, perhaps because of 
the drier climate, the air is less filled with smoke and 
haze in the southern portions of the African and Ameri- 
can continents than it is in our northern regions. 

What we have said of the diurnal motion of the 
northern stars round and round the pole, applies to the 
stars in the southern heavens. But there is no southern 
pole star, and therefore nothing to distinguish the posi- 
tion of the southern celestial pole. The latter has a 
number of small stars around it, but they are no thicker 
than in any other region of the sky. Of course, the 
southern hemisphere has its circle of perpetual appari- 
tion, which is larger the farther south we travel. That 
is to say, the stars in a certain circle around the south 
celestial pole never set, but simply revolve around it. 



18 THE CELESTIAL MOTIONS 

apparently in an opposite direction from what they do 
in the north. So, also, there is a circle of perpetual 
occultation containing those stars around the north pole 
which, in our latitudes, never set. After we go be3^ond 
^0° south latitude we can no longer see any part of the 
constellation Ursa Minor. Still farther south the Great 
Bear will only occasionally show itself to a greater or 
less extent above the horizon. 

Could we continue our journey to the south pole we 
should no longer see any rising or setting of the stars. 
The latter would move around the sky in horizontal 
circles, the centre or pole being at the zenith. Of course, 
the same thing would be true at the north pole. 



Ill 

Relation of Time and Longitude 

We all know that a line running through any place 
on the earth in a north and south direction, is called the 
meridian of that particular place. More exactly, a me- 
ridian of the earth's surface is a semicircle passing from 
the north to the south pole. Such semicircles pass in 
every direction from the north pole, and one may be 
drawn so as to pass through an}^ place. The meridian 
of the Royal Observator}^ at Greenwich is now adopted 
by most nations, our own included, as the one from which 
longitudes are measured, and by which in the United 
States and a considerable part of Europe the clocks 
are set. 

Corresponding to the terrestrial meridian of a place 
is a celestial meridian which passes from the north celes- 
tial pole through the zenith, intersects the horizon at its 
south point, and continues to the south pole. As the 
earth revolves on its axis it carries the celestial as well as 
the terrestrial meridian with it, so that the former, in 
the course of a day sweeps over the whole celestial 
sphere. The appearance to us is that every point of the 
celestial sphere crosses the meridian in the course of a 
day. 

Noon is the moment at which the sun passes the me- 
ridian. Before the introduction of railways, people used 



20 THE CELESTIAL MOTIONS 

to set their clocks by the sun. But owing to tlie obliquity 
of the ecliptic and the eccentricity of the earth's orbit 
around the sun, the intervals between successive passages 
of the sun are not exactly equaL The consequence is 
that, if a clock keeps exact time, the sun will sometimes 
pass the meridian before and sometimes after twelve by 
the clock. When this was understood, a distinction was 
made between apparent and mean time. Apparent time 
was the unequal time determined by the sun ; mean time 
was that given by a clock keeping perfect time month 
after month. The difference between these two is called 
the equation of time. Its greatest amounts are reached 
every year about the first of November and the middle 
of February. At the former time, the sun passes the 
meridian sixteen minutes before the clock shows twelve; 
in February, fourteen or fifteen minutes after twelve. 

To define mean time astronomers imagine a mean sun 
which always moves along the celestial equator so as to 
pass the meridian at exactly equal intervals of time, and 
which is sometimes ahead of the real sun and sometimes 
behind it. This imaginary or mean sun determines the 
time of day. The subject will perhaps be a little easier 
if we describe things as they appear, imagining the earth 
to be at rest while the mean sun revolves around it, cross- 
ing the meridian of every place in succession. We thus 
imagine noon to be constantly travelling around the 
world. In our latitudes, its speed is not far from a 
thousand feet per second; that is to say, if it is noon at 
a certain place where we stand, it will one second after- 
ward be noon about one thousand feet farther west, in 



STANDARD TIME SI 

another second a thousand feet yet farther west, and so 
on through the twent^'-four hours, until noon will once 
more get back where we are. The obvious result of this 
is that it is never the same time of day at the same mo- 
ment at two places east or west of each other. As we 
travel west, we shall continually find our watches to be 
too fast for the places which we reach, while in travelling 
east, they will be too slow\ This varying time is called 
local or astronomical time. The latter term is used be- 
cause it is the time determined by astronomical observa- 
tions at any place. 

Standard Time 

Formerly the use of local time caused great inconven- 
ience to travellers. Every railway had its own meridian 
which it ran its trains by; and the traveller was fre- 
quently liable to miss his train by not knowing the rela- 
tion betw^een his w^atch or a clock and the railway time. 
So in 1883, our present system of standard time was in- 
troduced. Under this system, standard meridians are 
adopted fifteen degrees apart, this being the space over 
which the sun passes in one hour. The time at which 
noon passes a standard meridian is then used throughout 
a zone extending seven or eight degrees on each side. 
This is called standard time. The longitudes which 
mark the zones are reckoned from Greenwich. It hap- 
pens that Philadelphia is about seventy-five degrees in 
longitude, or five hours in time from Greenwich. More 
exactly, it is about one minute of time more than this. 
Thus the standard meridian which w^e use for the Middle 



22 THE CELESTIAL MOTIONS 

States passes a little east of Philadelphia. When mean 
noon reaches this meridian, it is considered as twelve 
o'clock throughout all our Eastern and Middle States 
as far west as Ohio. An hour later, it is considered twelve 
o'clock in the INIississippi Valley. An hour later, it is 
twelve o'clock for the region of the Rocky Mountains. 
In yet another hour, it is twelve o'clock on the Pacific 
coast. Thus we use four different kinds of time. Eastern 
time. Central time. Mountain time, and Pacific time, dif- 
fering from each other by entire hours. Using this time, 
the traveller only has to set his watch forward or back 
one hour at a time, as he travels between the Pacific and 
the Atlantic coast, and he will always find it correct for 
the region in which he is at the time. 

It is by this diff'erence of time that the longitudes of 
places are determined. Imagine that an observer in 
New York makes a tap with a telegraph-key at the exact 
moment when a certain star crosses his meridian, and 
that this moment is recorded at Chicago as well as New 
York. When the star reaches the meridian of Chicago, 
the observer taps the time of its crossing over his meri- 
dian in the same way. The interval between the two 
taps shows the diff'erence of longitude between the two 
cities. 

Another method of getting the same result is for each 
observer to telegraph his local time to the other. The 
difference of the two times gives the longitude. 

In this connection, it must be remembered that the 
heavenly bodies rise and set by local, not standard, time. 
Hence the time of rising and setting of the sun, given in 



WHERE THE DAY CHANGES S3 

the almanacs, will not answer to set our watches by for 
standard time, unless we are on one of the standard 
meridians. One difference between these two kinds of 
time is that local time varies continuously as we travel 
east or west, while standard time varies only by jumps 
of one hour when we cross the boundaries of any of the 
four zones just described. 

Where the Day Changes 

jMidnight, like noon, is continually travelling round 
the earth, crossing all the meridians in succession. At 
every crossing it inaugurates the beginning of another 
day on that meridian. If it is Monday at any crossing, 
it will be Tuesday when it gets back again. So there 
must be some meridian where Monday changes to Tues- 
day, and where every day changes into the day follow- 
ing. This dividing meridian, called the "date line," is 
determined only by custom and convenience. As colo- 
nization extended toward the east and the west men 
carried their count of days with them. The result was 
that whenever it extended so far that those going east 
met those going west they found their time differing by 
one day. What for the westward traveller was Monday 
was Tuesday for the eastern one. This was the case 
when we acquired Alaska. The Russians having reached 
that region by travelling east, it was found that, when 
we took possession by going west, our Saturday was their 
Sunday. This gave rise to the question whether the 
inhabitants, in celebrating the festivals of the Greek 
Church, should follow the old or the new reckoning of 



24 THE CELESTIAL MOTIONS 

days. The subject was referred to the head of the church 
at St. Petersburg, and finally to Struve, the director of 
the Pulkowa Observatory, the national astronomical in- 
stitution of the empire. Struve made a report in favor 
of the American reckoning, and the change to it was 
duly carried out. 

At the present time custom prescribes for the date line 
the meridian opposite that of Greenwich. This passes 
through the Pacific Ocean, and in its course crosses very 
little land — only the northeastern corner of Asia and, 
perhaps, some of the Fiji Islands. This fortunate cir- 
cumstance prevents a serious inconvenience which might 
arise if the date line passed through the interior of a 
country. In this case the people of one city might have 
their time a day different from those of a neighbouring 
city across tlie line. It is even conceivable that residents 
on two sides of the same street would have different days 
for Sunday. But being in the ocean, no such incon- 
venience follows. The date line is not necessarily a meri- 
dian of the earth, but may deviate from one side to the 
other in order to prevent the inconvenience we have 
described. Thus the inhabitants of Chatham Island 
have the same time as that of the neighbouring island of 
New Zealand, although the meridian of 180° from 
Greenwich runs between them. 



IV 

How THE Position of a Heavenly Body is Defined 

In this chapter I have to use and explain some tech- 
nical terms. The ideas conveyed by them are necessary 
to a complete understanding of the celestial motions, and 
of the positions of the stars at any hour when we may 
wish to observe them. To the reader who only desires 
a general idea of celestial phenomena, this chapter will 
not be necessary. I must invite one who wants a knowl- 
edge more thorough than this to make a close study of 
the celestial sphere as it was described in our second 
chapter. Turning back to our first figure, we see our- 
selves concerned Avith the relation of two spheres. One 
of these is the real globe of the earth, on the surface of 
which we dwell, and which is continually carrying us 
around by its daily rotation. The other is the apparent 
sphere of the heavens, which surrounds our globe on all 
sides at an enormous distance, and which, although it has 
no reality, we are obliged to imagine in order to know 
where to look for the heavenly bodies. Notice that we 
see this sphere from its centre, so that everything we 
see upon it appears upon its inside surface, while we see 
the surface of the earth from the outside. 

There is a correspondence between points and circles 
on these two spheres. We have already shown how the 
axis of the earth, which marks our north and south poles, 



26 THE CELESTIAL MOTIONS 

being continued in both directions through space, marks 
the north and south poles of the celestial sphere. 

We know that the earth's equator passes around it at 
an equal distance from the two poles. In the same way 
we have an equator on the celestial sphere which passes 
around it at a distance of ninety degrees from either 
celestial pole. If it could be painted on the sky we should 
always see it, b}^ day or night, in one fixed position. We 
can imagine exactly how it would look. It intersects the 
horizon in the east and west points, and is in fact the line 
which the sun seems to mark out in the sky by its diurnal 
course during the twelve hours that it is above the hori- 
zon, in March or September. In our northernmost 
States, it passes about halfway between the zenith and 
the south horizon, but passes nearer the zenith the farther 
south we are. 

As we have circles of latitude parallel to the equator 
passing around the earth both north and south of the 
equator, so we have on the celestial sphere circles parallel 
to the celestial equator, and therefore having one or the 
other of the celestial poles as a centre. As the parallels 
of latitude on the earth grow smaller and smaller toward 
the pole, so do these celestial circles grow smaller toward 
the celestial poles. 

We know that longitude on the earth is measured by 
the position of a meridian passing from the north to the 
south pole through the place whose position is to be de- 
fined. The angle which this meridian makes with that 
through the Greenwich Observatory is the longitude of 
the place. 



CIRCLES OF CELESTIAL SPHERE 



27 



We have the same system in the heavens. Circles are 
imagined to pass from one celestial pole to the other in 
every direction, but all intersecting the equator at right 







Fig. 3. — Circles of the Celestial Spin 



angles, as shown in Figure 3. These are called hour 
circles. One of them is called the first hour circle, and 
is so marked in the figure. It passes through the vernal 



28 THE CELESTIAL MOTIONS 

equinox, a point to be defined in the next chapter. This 
takes a place in the sky corresponding to Greenwich on 
the earth's surface. 

The position of a star on the celestial sphere is defined 
in the same way that the position of a city on the earth 
is defined, by its latitude and longitude. But different 
terms are used. In astronomy, the measure which corre- 
sponds to longitude is called right ascension; that which 
corresponds to latitude is called declination. We thus 
have the following definitions, which I must ask the 
reader to remember carefull}^ 

The declination of a star is its apparent distance from 
the celestial equator north or south. In the figure the 
star is in declination twentj^-five degrees north. 

The right ascension of a star is the angle which the 
hour circle passing through it makes with the first hour 
circle which passes through the vernal equinox. In the 
figure the star is in three hours right ascension. 

The right ascension of a star is, in astronomical usage, 
generally expresssed as so many hours, minutes, and 
seconds, in the w^ay shown on Figure 3. But it may 
equally well be expressed in degrees as we express the 
longitude of places on the earth. The right ascension 
expressed in hours may be changed into degrees by the 
simple process of multiplication by 15. This is because 
the earth resolves 15° in an hour. Figure S also shows 
us that, while the degrees of latitude are nearl^^ of 
the same length all over the earth, those of longitude 
continually diminish, slowly at first and more rapidly 
afterwards, from the equator toward the poles. At the 



POSITION OF A HEAVENLY BODY 29 

equator the degree of longitude is about 69| statute miles, 
but at the latitude of 45° it is only about 42 miles. At 
60*^ it is less than 35 miles, at the pole it comes down to 
nothing, because there the meridians meet. 

We may see that the speed of the rotation of the 
earth follows the same law of diminution. At the equa- 
tor, 15° is about 1,000 miles. We may therefore see 
that, in that part of the earth, the latter revolves at 
the rate of 1,000 miles an hour. This is about 1,500 
feet per second. But in latitude 45° the speed is 
diminished to little more than 1,000 feet per second. 
At 60°, north, it is only half that at the equator ; at the 
poles it goes down to nothing. 

In applying this system the only trouble arises from 
the earth's rotation. As long as we do not travel, we 
remain on the same circle of longitude on the earth. But 
by the rotation of the earth, the right ascension of any 
point in the sky which seems to us fixed, is continually 
changing. The only difference between the celestial 
meridian and an hour circle is that the former travels 
round with the earth, while the latter is fixed on the 
celestial sphere. 

There is a strict resemblance in almost every point 
betw^een the earth and the celestial sphere. As the former 
revolves on its axis from west to east, the latter seems to 
revolve from east to west. If we imagine the earth cen- 
tred inside the celestial sphere with a common axis pass- 
ing through them, as shown in the figure, we shall have a 
clear idea of the relations we wish to set forth. 

If the sun, like the stars, seemed fixed on the celestial 



30 THE CELESTIAL MOTIONS 

sphere from 3^ear to 3'ear, the problem of finding a star 
wlien we knew its right ascension and declination would 
be easier than it actually is. Owing to the annual revo- 
lution of the earth round the sun there is a continual 
change in the apparent position of the sphere at a given 
hour of the night. We must next point out the effect 
of this revolution. 



V 

The Annual Motion of the Earth and its Results 

It is well known that the earth not only turns on its 
axis, but makes an annual revolution round the sun. The 
result of this motion — in fact, the phenomenon by which 
it is shown — is that the sun appears to make an an- 
nual revolution around the celestial sphere among the 
stars. We have only to imagine ourselves moving round 
the sun and therefore seeing the latter in different direc- 
tions, to see that it must appear to us to move among 
the stars, which are farther than it is. It is true that 
the motion is not at once evident because the stars are 
invisible in the daytime. But the fact of the motion 
will be made very clear if, day after day, we watch some 
particular fixed star in the west. We shall find that it 
sets earlier and earlier every day ; in other words, it is 
getting continually nearer and nearer the sun. More 
exactly, since the real direction of the star is unchanged, 
the sun appears to be approaching the star. 

If we could see the stars m the daytime, all round the 
sun, the case would be yet clearer. We should see that 
if the sun and a star rose together in the morning the 
sun would, during the day, gradually work past the star 
in an easterly direction. Between the rising and setting 
it would move nearly its own diameter relative to the star. 
Next morning we should see that it had gotten quite 



32 THE CELESTIAL MOTIONS 

away from the star, being nearly two diameters distant 
from it. The figure shows how this would go on at the 
time of the spring equinox, after March twentieth. This 
motion would continue month after month. At the end 




Fig. 4. — The Sun Crossing the Equator about March Twentieth, 



of the year the sun would have made a complete circuit 
of the heavens relative to the star, and we should see the 
two once more together. 

The Sun^s Apparent Path 

How the above effect is produced will be seen by Fig- 
ure 5, which represents the earth's orbit round the sun, 
with the stars in the vast distance. When the earth is at 
A, we see the sun in the line AM, as if it were among the 
stars at M. As we are carried on the earth from A to B, 
the sun seems to move from M to N, and so on through 
the year. This apparent motion of the sun in one year 
around the celestial sphere, was noticed by the ancients, 
who seem to have taken much trouble to map it out. They 



THE SUN'S APPARENT PATH 



33 



imagined a line passing around the celestial sphere which 
the sun alwa3^s followed in its annual course, and which 
was called the ecliptic. They noticed that the planets 
followed nearly but not exactly the same general course 
as the sun among the stars. A belt extending around on 





^.o^^'J^Wj^'^.o 




V * ■♦^ "" 1 ' •* » ^/ 

y* ~^. 

* \ 1 / *v 




» » 

** 
* 

r * 


! / .-^ **. 




1 \ ^--** 




/ 1 \ ♦o" 


** 


K 







-M 



Fig. 5. — J7ie Orbit of the Earth and the Zodiac. 

each side of the ecliptic, and broad enough to contain 
all the known planets, as well as the sun, was called the 
zodiac. It was divided into twelve signs, each marked 
by a constellation. The sun went through each sign in 
the course of a month and through all twelve signs in a 
year. Thus arose the familiar signs of the zodiac, which 



34 THE CELESTIAL MOTIONS 

bore the same names as the constellations among which 
they were situated. This is not the case at present, 
owing to the slow motion of precession soon to be 
described. 

It will be seen that the two great circles we have de- 
scribed spanning the entire celestial sphere are fixed in 
entirely different ways. The equator is determined by 
the direction in which the axis of the earth points, and 
spans the sphere midway between the two celestial poles. 
The ecliptic is determined by the earth's motion around 
the sun. 

These two circles do not coincide, but intersect each 
other at two opposite points, at an angle of twenty-three 
and a half degrees, or nearly one quarter of a right 
angle. This angle is called the obliquity of the ecliptic. 
To understand exactly how it arises we must mention 
a fact about the celestial poles ; from what we have said 
of them it will be seen that they are not determined by 
anything in the heavens, but by the direction of the 
earth's axis only; they are nothing but the two op- 
posite points in the heavens which lie exactly in the 
line of the earth's axis. The celestial equator, being 
the great circle halfway between the poles, is also 
fixed by the direction of the earth's axis and by nothing 
else. 

Let us now suppose that the earth's orbit around the 
sun is horizontal. We may in imagination represent 
it by the circumference of a round level platform with 
the sun in its centre. We suppose the earth to move 
around the circumference of the platform with its cen- 



THE SUN'S APPARENT PATH 35 

tre on the level of the platform ; then, if the earth's axis 
were vertical, its equator would be horizontal and on a 
level with the platform and therefore would always be 
directed toward the sun in its centre, as the earth made its 
annual course around the platform. Then, on the celes- 
tial sphere, the ecliptic determined by the course of the 
sun would be the same circle as the equator. The 
obliquity of the ecliptic arises from the fact that the 
earth's orbit is not vertical, as just supposed, but is in- 




FiG. 6. — How the Obliquity of the Ecliptic Produces the Changes of Seasons. 

clined twenty-three and a half degrees. The ecliptic 
has the same inclination to the plane of the platform; 
thus the obliquity is the result of the inclination of the 
earth's axis. An important fact connected with the sub- 
ject is that, as the earth makes its revolutions around the 
sun, the direction of its axis remains unchanged in space ; 
hence its north pole is tipped away from the sun or 
toward it, according to its position in the orbit. This 
is shown in Figure 6, which represents the platform we 
have supposed, with the axis tipped toward the right 
hand. The north pole will always be tipped in this 



36 



THE CELESTIAL MOTIONS 



direction, whether the earth is east, west, north, or south 
from the sun. 

To see the effect of the inchnation upon the echptic 
suppose that, at noon on some twenty-first day of March, 
the earth should suddenly stop turning on its axis, but 
continue its course around the sun. What we should then 
see during the next three months is represented in Figure 
7, in which we are supposed to be looking at the southern 
sky. We see the sun on the meridian, where it will at 
first seem to remain immovable. The figure shows the 




Fig. V. — Apparent Motion of the Sun along the Ecliptic in Sjyring and 
Summer. 



celestial equator passing through the east and west 
points of the horizon as already described and also the 
ecliptic, intersecting it at the equinox. Watching the 
result for a time equal to three of our months we should 
see the sun slowly make its way along the ecliptic to 
the point marked "summer solstice," its farthest north- 
ern point, which it would reach about June twentieth. 

Figure 8 enables us to follow its course for three 
months longer. After passing the summer solstice, its 



THE SUN'S APPARENT PATH 



37 



course gradually carries it once more to the equator, 
which it again crosses about September twentieth. Its 
course during the rest of the j'ear is the counterpart of 
that during the first six months. It is farthest south of 
the equator on December twentieth, and again crosses it 
on March twentieth. 

We see that there are four cardinal points in this ap- 
parent annual course of the sun. (1) Where we have 
commenced our watch is the vernal equinox. (2) The 
point where the sun, having reached its northern limit, 
begins to again approach the equator. This is called the 
summer solstice. ( 3 ) Opposite the vernal equinox is the 











a. 

z 


lU 

l 














"'''^"n.J^ 




^ 


_R^ 






_e 


Q^ 


^ 


^ 
< 


5% 


^v,^° 




< 

1- 






f 


o / y^ 

"iA^r . H O R 1 Z 


o 


N 




UJ 

a: 

_l 

O 






H_ 


^ 


R 1 Z ON 


. \ z 



Fig. 8. — Apparent Motion of the San from March till September. 

autumnal equinox, which the sun passes about September 
twentieth. (4) Opposite the summer solstice is the point 
where the sun is farthest south. This is called the winter 
solstice. 

The hour circles which pass from one celestial pole to 
the other through these points at right angles to the 
equator are called colures. That which passes through 



38 THE CELESTIAL MOTIONS 

the vernal equinox is the first meridian, from which right 
ascensions are counted as ah^eadj described. The two 
at right angles to it are called the solstitial colures. 

Let us now show the relation of the constellations to 
the seasons and the time of day. Suppose that to-day 
the sun and a star passed the meridian at the same mo- 
ment; to-morrow the sun will be nearly a degree to the 
east of the star, which shows that the star will pass the 
meridian nearly four minutes sooner than the sun will. 
This will continue day after day throughout the entire 
year when the two will again pass the meridian at about 
the same moment. Thus the star will have passed once 
oftener than the sun. That is to say: In the course 
of a year while the sun has passed the meridian three 
hundred and sixty-five times, a star has passed it three 
hundred and sixty-six times. Of course if we take a 
star in the south it will have risen and set the same 
number of times. 

Astronomers keep the reckoning of this different ris- 
ing and setting of the stars by using a sidereal day, or 
star day, equal to the interval between two passages of 
a star, or of the vernal equinox, across the meridian. They 
divide this day into twenty-four sidereal hours, and these 
into minutes and seconds according to the usual plan. 
They also use sidereal clocks which gain about three 
minutes and fifty-six seconds per day on the ordinary 
clocks, and thus show sidereal time. Sidereal noon is the 
moment at which the vernal equinox crosses the meridian 
of the place. The clock is then set at hours, minutes, 
and seconds. Thus set and regulated, the sidereal 



THE SEASONS 39 

clock keeps time with the apparent rotation of the celes- 
tial sphere, so that the astronomer has only to look at his 
clock to see, b}^ day or by night, what stars are on the 
meridian and what the positions of the constellations are. 

The Seasons 

If the earth's axis were perpendicular to the plane of 
the ecliptic, the latter would coincide with the equator, 
and we should have no difference of seasons the year 
round. The sun would always rise in the exact east and 
set in the exact west. There would be only a very slight 
change in the temperature arising from the fact that 
the earth is a little nearer the sun in January than in 
July. Owing to the obliquity of the ecliptic it follows 
that, while the sun is north of the equator, which is the 
case from March to September, the sun shines upon the 
northern hemisphere during a greater time of each day 
and at a greater angle, than on the southern hemisphere. 
In the southern hemisphere the opposite is the case. The 
sun shines longer from September till March than it does 
on the northern hemisphere. Thus we have winter in 
the northern hemisphere when it is summer in the 
southern, and vice versa. 

Relations between Real and Apparent Motions 

Before going farther let us recapitulate the phenom- 
ena we have described from the two points of view: one 
that of the real motions of the earth; the other that of 
the apparent motions of the heavens, to which the real 
motions give rise. 



4^0 THE CELESTIAL MOTIONS 

The real diurnal motion is the turning of the earth on 
its axis. 

The apj3arent diurnal motion is that which the stars 
appear to have in consequence of the earth's rotation. 

The real annual motion is that of the earth round the 
sun. 

The apparent annual motion is that of the sun around 
the celestial sphere among the stars. 

By the real diurnal motion the plane of our horizon is 
carried past the sun or a star. 

We then sa}^ that the sun or star rises or sets, as the 
case may be. 

About March twenty-first of every year the plane of 
the earth's equator passes from the north to the south 
of the sun, and about September twenty-first it repasses 
toward the north. 

We then say that the sun crosses to the north of the 
equator in iMarch, and to the south in September. 

In June of every ^^ear the plane of the earth's equator 
is at the greatest distance south of the sun, and in De- 
cember at the greatest distance north. 

We say in the first case that the sun is at the north- 
ern solstice, and in the second that it is at the southern 
solstice. 

The earth's axis is tipped twenty-three and a half 
degrees from the perpendicular to the earth's orbit. 

The apparent result is that the ecliptic is inclined 
twenty-three and a half degrees to the celestial equator. 

During June and the other summer months the north- 
ern hemisphere of the earth is tipped toward the sun. 



PRECESSION OF THE EQUINOXES 41 

Places in north latitude, as they are carried round by 
the turning of the earth, are then in sunlight during 
more than half their course ; those in south latitude less. 

The result as it appears to us is that the sun is more 
than half the time above the horizon, and that we have 
the hot weather of summer, while in the southern hemi- 
sphere the days are short, and the season is winter. 

During our winter months the case is reversed. The 
southern hemisphere is then tipped toward the sun, and 
the northern hemisphere away from it. Consequently, 
summer and long days are the order in the southern, and 
the reverse in the northern hemisphere. 

The Year and the Precession of the Equinoxes 

We most naturally define the year as the interval of 
time in which the earth revolves around the sun. From 
what we have said, there are two ways of ascertaining its 
length. One is to find the interval between two passages 
of the sun past the same star. The other is to find the 
interval between two passages of the sun past the same 
equinox, that is, across the equator. If the latter were 
fixed among the stars the two intervals would be equal. 
But it was found by the ancient astronomers, from obser- 
vations extending through several centuries, that these 
two methods did not give the same length of year. It 
took the sun about eleven minutes longer to make the 
circuit of the stars than to make the circuit of the 
equinoxes. This shows that the equinoxes steadily 
shift their position among the stars from year to year. 
This shift is called the precession of the equinoxes. It 



42 THE CELESTIAL MOTIONS 

does not arise from anything going on in the heavens, but 
onl}^ from a slow change in the direction of the earth's 
axis from year to year as it moves around the sun. 

If we should suppose the platform in Figure 6 to last 
for six or seven thousand years, and the earth to make its 
six or seven thousand revolutions around it, we should 
find that, at the end of this time, the north end of the 
axis of the earth, instead of being tipped toward our 
right hand, as shown in the figure, would be tipped 
directly toward us. At the end of another six or seven 
thousand years it would be tipped toward our left ; at the 
end of a third such period it would be tipped away from 
us, and at the end of a fourth, or about twenty-six 
thousand years in all, it would have gotten back to its 
original direction. Since the celestial poles are deter- 
mined by the direction of the earth's axis, this change in 
the direction of the axis makes them slowly go around a 
circle in the heavens, having a radius of about twenty- 
three and a half degrees. At the present time the pole 
star is a Kttle more than a degree from the pole. But 
the pole is gradually approaching it and will pass by it 
in about two hundred years. In twelve thousand years 
from now the pole will be in the constellation Lyra, about 
five degrees from the bright star Vega of that constella- 
tion. In the time of the ancient Greeks their navigators 
did not recognize any pole star at all, because what is 
now such was then ten or twelve degrees from the pole, 
the latter having been between it and the constellation of 
the Great Bear. It was the latter which they steered by, 
and which they called the Cynosure. 



LENGTH OF THE YEAR 43 

It follows from all this that, since the celestial equator 
is the circle midway between the two poles, there must 
be a corresponding shift in its position among the stars. 
The effect of this shift during the past two thousand 
years is shown in Figure 9. Since the equinoxes are the 
points of crossing of the ecliptic and the equator, they 
also change in consequence of this motion. It is thus 
tliat the precession of the equinoxes arises. 



* O^ 2000 YEARS AGO 



> CELESTIAL EQUATOq 2000 YEARS AGO 
CONSTELLjATION PISCES 

CELESTIAL EQUATOR NOW -^-^O^ 



Fig. 9. — Precession of the Equinoxes. 

The two kinds of year we have described are called 
equinoctial and sidereal. The equinoctial year, also 
called the solar year, is the interval between two returns 
of the sun to the equinox. Its length is — 

365 days 5 hours 48 minutes 46 seconds. 

Since the seasons depend upon the sun's being north 
or south of the equator, the solar or equinoctial year is 
that used in the reckoning of time. The ancient astrono- 
mers found that its length was about three hundred 
and sixty-five and one quarter days. As far back as the 
time of Ptolemy the length of the year was known even 
more exactly than this, and found to be a few minutes 
less than three hundred and sixty-five and one quarter 
days. The Gregorian Calendar, which nearly all civi- 



U THE CELESTIAL MOTIONS 

lised nations now use, is based upon a close approxima- 
tion to this length of the jenr. 

The sidereal 3' ear is the interval between two passages 
of the sun past the same star. Its length is three hun- 
dred and sixty -five days six hours and nine minutes. 

According to the Julian calendar, which was in use 
in Christendom until 1582, the year was considered to 
be exactly 365 1 days. This, it will be seen, was 11 
minutes 14 seconds more than the true length of the 
solar year. Consequently, the seasons were slowly 
changing in the course of centuries. In order to obviate 
this, and have a year whose average length was as nearly 
as possible correct, a decree was passed by Pope Gregory 
XIII b}^ which, in three centuries out of four, a day 
was dropped from the Julian calendar. According to 
the latter, the closing year of every century would be 
a leap year. In the Gregorian calendar 1600 was still 
to remain a leap year, but 1500, 1700, 1800, and 1900 
were all common years. 

The Gregorian calendar was adopted immediately by 
all Catholic countries, and from time to time by Protes- 
tant countries also, so that for the past 150 years it has 
been universal in both. But Russia has held on to the 
Julian calendar until this day. Consequently in that 
country the reckoning of time is now 13 days behind 
that in the other Christian countries. The Russian New 
Year of 1900 occurred on what we call January 13. In 
February of that year we only counted 28 days, but 
Russia counted 29. Hence, in 1901, the Russian New 
Year was carried still farther forward to our January 14. 



PART II 
ASTRONOMICAL INSTRUMENTS 



I 

The Refracting Tei^escope 

There is no branch of science more interesting to the 
pubhc than that with which the telescope is concerned. I 
assume that the reader wishes to have an intelhgent idea 
as to what a telescope is and what can be seen with it. 
In its most complete form, as used by the astronomer in 
his observatory, the instrument is quite complex. But 
there are a few main points about it which can be mas- 
tered in a general way by a little close attention. After 
mastering these points, the visitor to an observatory will 
examine the instrument with much more satisfaction than 
he can when he knows nothing about it. 

The one great function of a telescope, as we all know, 
is to make distant ob j ects look nearer to us ; to see an 
object miles away as if it were, perhaps, only as many 
3"ards. The optical appliances by which this is effected 
are extremely simple. They are made with large well- 
polished lenses, of the same kind as those used in a pair 
of spectacles, differing from the latter only in their size 
and general perfection. A telescope requires an ap- 
pliance for collecting the light coming from the object 
so as to form an image of the latter. There are two 
ways in which the light may be collected, one by passing 
the light through a set of lenses, and one by reflecting it 
from a concave mirror. Thus we have two different kinds 



48 ASTRONOMICAL INSTRUMENTS 

of telescope, one called refracting, the other reflecting. 
We begin with the former because it is the more usual. 

The Lenses of a Telescope 

The lenses of a refracting telescope comprise two com- 
binations or systems; the one an object-glass — or "ob- 
jective," as it is sometimes called for shortness — which 
foiTQs the image of a distant object in the focus of the 
instrument ; and the other an eyepiece, with which this 
image is viewed. 

The objective is the really difficult and delicate part 
of the instrument. Its construction involves more refined 
skill than that of all the other parts together. How great 
is the natural aptitude required may be judged from the 
fact that a generation ago there was but one man in the 
world in whose ability to make a perfect object-glass of 
the largest size astronomers everywhere would have felt 
confidence. This man was Alvan Clark, of whom we 
shall soon speak. 

The object-glass, as commonly made, consists of two 
large lenses. The power of the telescope depends alto- 
gether on the diameter of these lenses, which is called 
the aperture of the telescope. The aperture may vary 
from three or four inches, in the little telescope which 
one has in his house, to more than three feet in the great 
telescope of the Yerkes Observatory. One reason why 
the power of the telescope depends on the diameter 
of the object-glass is that, in order to see an object mag- 
nified a certain number of times, in its natural bright- 
ness, we need a quantity of light expressed hy the square 



THE LENSES OF A TELESCOPE 49 

of the magnifying power. For example, if we have a 
magnifying power of one hundred, we should need ten 
thousand times the light. I do not mean that this quan- 
tity of light is always necessary ; it is not so, because we 
can commonly see an object with less than its natural 
illumination. Still, we need a certain amount of light, 
or it will be too dim. 

In order that distinct vision of a distant object may 
be secured in the telescope, the one great essential is that 
the object-glass should bring all the rays coming from 
any one point of the object observed to the same focus. 
If this is not brought about; if different rays come to 
slightly different foci, then the object will look blurred, 
as if it were seen through a pair of spectacles which did 
not suit our eyes. Now, a single lens, no matter of what 
sort of glass we make it, will not bring rays to the same 
focus. The reader is doubtless aware that ordinary light, 
whether coming from the sun or a star, is of a countless 
multitude of different colours, which can be separated by 
passing the light through a triangular prism. These 
colours range from red at one end of the scale, through 
yellow, green, and blue, to violet at the other. A single 
lens brings these different rays to different foci ; the red 
farthest from the object-glass; the violet nearest to it. 
This separation of the rays is called dispersion. 

The astronomers of two centuries ago found it impos- 
sible to avoid the dispersion of a lens. iVbout 1750, 
Dollond, of London, found that it was possible to cor- 
rect this defect by using two different kinds of glass, 
the one crown glass and the other flint glass. The prin- 



50 ASTRONOMICAL INSTRUMENTS 

ciple by which this is done is very simple. Crown glass 
has nearly the same refracting power as flint, but it has 
nearly twice the dispersive power. So Dollond made 
an objective of two lenses, a section of which is shown in 
the figure. First there was a convex lens of crown glass, 
which is of the usual construction. Combined with this 
is a concave lens of flint glass. These two lenses, being 
of opposite curvatures, act on the light in opposite direc- 
tions. The crown glass tends to bring the light to a 
focus, while the flint, being concave, would make the rays 
diverse. If it were used alone, we 

.ZhXUJ G LAS S ,-,,«,. 

^^^^^^^^ should find that the rays passmg 

CROWN GLASS througli it, instead of coming to a 

Fig. lo.-Sedion of the focus, diverge farther and farther 

Object.glass of a Tele- f^.^^ g^ f^ ^^ different direc- 

scope. 

. tions. Now, the flint glass is made 

with but little more than half the power of the crown. 
This half power is sufficient to neutralize the dispersion 
of the crown ; but it does not neutralize much more than 
half the refraction. The combined result is that all the 
rays passing through the combination are brought nearly 
to one focus, which is about twice as far away as the 
focus of the crown alone. 

I say brought nearly to one focus. It happens, un- 
fortunately, that the combined action of the two glasses 
is such that it is impossible to bring all the rays of the 
various colours absolutely to the same focus. The diver- 
gence, in the case of the brighter rays, can be made very 
small indeed, but it cannot be cured entirely. The larger 
the telescope, the more serious the defect. If you look 



THE IMAGE OF A DISTANT OBJECT 51 

at a bright star through any large refracting telescope, 
you will see it surrounded by a blue or purple radiance. 
This is produced by the blue or violet light which the 
two lenses will not bring to one focus. 

The Image of a Distant Object 

By the action of the objective, in thus bringing rays 
to a focus, the image of a distant object is formed in 
the focal plane. This is a plane passing through the 
focus at right angles to the axis or line of sight of the 
telescope. 

What is meant by the image formed by a telescope can 
be seen by looking into the ground glass of a camera with 
the photographer, as he sets his instrument for a picture. 
You there see a face or a distant landscape pictured on 
the ground glass. To all intents and purposes the 
camera is a small telescope, and the ground glass, or the 
point where the sensitive plate is to be fixed to take a 
picture, is the focal plane. We may state the matter 
in the reverse direction by saying that the telescope is a 
large camera of long focus, with which we can take 
photographs of the heavens as the photographer takes 
ordinary pictures with the camera. 

Sometimes we can better comprehend what an object 
is by understanding what it is not. In the celebrated 
moon hoax of half a century ago or more, there was a 
statement which illustrates what an imaece is not. The 
writer said that Sir John Herschel and his friend finding 
that, when they used enormous magnifying power, there 
was not light enough for the image to be visible, the 



52 ASTRONOMICAL INSTRUMENTS 

friend suggested that tlie image should be illuminated 
by artificial light. This was done with such brilliant 
success that animals in the moon were made visible 
through the telescope. If many people, even those of 
the greatest intelligence, had not been deceived by this, 
I should hardly deem it necessary to say that the image 
of an object formed by a telescope is such that, in the 
very nature of things, extraneous light cannot aid in its 
formation. Its effectiveness does not proceed from its 
being a real image, but only from the fact that all the 
rays from any one point of a distant object meet in a 
corresponding point of the image, and there diverge 
again, just as if a picture of the object were placed in 
the focal plane. The fact is that the term picture is 
perhaps a little better one than image to apply to this 
representation of the object, only the picture is formed 
by light and nothing else. 

If an image or picture of the object is thus formed 
so as to stand out before our eyes, one may ask why an 
eyepiece is necessary to view it ; why the observer cannot 
stand behind the picture, look toward the objective and 
see the picture hanging in the air, as it were. He can 
really do so if he holds a ground glass in the focal plane, 
as the photographer does with the camera. He can thus 
see the image formed on the glass. If he looks into the 
object-glass he can see it without any eyepiece. But 
only a very small portion of it will be visible at any one 
point, and the advantage over looking directly at the 
object will be slight. To see it to advantage an eyepiece 
must be used. This is nothing more than a little eye 



POWER AND DEFECTS OF TELESCOPE 53 

glass, essentially of the same kind that the watchmaker 
uses to examine the works of a watch. The smaller the 
eyepiece, the more closely the examination can be made, 
and the greater the magnifying power. 

Power and Defects of a Telescope 

The question is often asked, how great is the magnify- 
ing power of some celebrated telescope. The answer is 
that the magnifying power depends not only on the 
object-glass but on the eyepiece. The smaller the latter 
the greater the magnifying power. Astronomical tele- 
scopes are supplied with quite a large collection of eye- 
pieces, varying from the lowest to the highest power, 
according to the needs of the observer. 

So far as the geometric principle goes, we can get any 
magnifying power we please on any telescope, however 
small. By viewing the image with an ordinary micro- 
scope, such as is used by physicians, we might give a 
little four-inch telescope the magnification of Herschel's 
great reflectors. But there are many practical difficul- 
ties in carrying the magnification of any instrument 
above a certain point. First there is the want of light 
in seeing the surface of an object. If we looked at 
Saturn with a three-inch telescope, using a magnifying 
power of several hundred times, the planet would seem 
dim and indistinct. But this is not the only difficulty 
in using a high magnifying power with a small telescope. 
The effect of light having a wave length is such that 
as a general rule we can get no advantage in carrying 
the magnification above fifty, or one hundred at the 



54 ASTRONOMICAL INSTRUMENTS 

most, for each inch of aperture. That is to say, with a 
three-inch telescope we should gain no advantage by 
using a power much above one hundred and fifty, and 
certainly none above three hundred. 

But a large telescope also has its defects, owing to 
the impossibility of bringing all the light to absolutely 
the same focus. There is a limit to the magnification 
which can be used, rather difficult to define exactly, but 
of which the observer will be very sensible when he looks 
into the instrument and sees the blue aureole already 
mentioned. 

But there is still another trouble, which annoys the 
astronomer more than all others, but which the public 
rarely understands. 

We see a heavenly body through a thickness of atmos- 
phere which, were it all compressed to the density that 
it has around us, would be equal to about six miles. We 
know that when we look at a body six miles away, we 
see its outlines softened and blurred. This is mainly 
because the atmosphere through w^hich the rays have to 
pass is constantly in motion, thus producing an irregular 
refraction which makes the body look wavy and tremu- 
lous. The softened and blurred effect thus produced is 
magnified in a telescope as many times as the object 
itself. The result is that as we increase the magnify- 
ing power we increase a certain indistinctness in the 
vision in the same proportion. The amount of this 
indistinctness depends very much on the condition of 
the air. The astronomer having this in mind tries to 
find a perfectly clear air, or, rather, air which is very 



MOUNTING OF THE TELESCOPE 55 

stead}', so that the heavenly bodies will look sharp when 
seen through it. 

We frequently see calculations showing how near the 
moon can be brought to us by using some high magnif}^- 
ing power. For example, with a power of one thou- 
sand we see it as if it were two hundred and forty miles 
away ; with about five thousand, as if it were forty-eight 
miles awa}^ This calculation is quite correct so far as 
the apparent size of any object on the moon is concerned, 
but it takes no account either of the imperfections of the 
telescope or the bad effect produced by the atmosphere. 
The result of both of these defects is that such calcula- 
tions do not give a correct idea of the truth. I doubt 
whether any astronomer with any telescope now in exist- 
ence could gain a great advantage, in the study of such 
an object as the moon or a planet, by carrying his mag- 
nification above a thousand, unless on very rare occa- 
sions in an atmosphere of unusual stillness. 

Mounting of the Telescope 

Those who have never used a telescope are apt to think 
that the work of observing with it is simply to point it at 
a heavenly body and examine the latter through it.* 

* The writer recalls that when Mr. James Lick was founding 
the observatory which has since become so celebrated, the great 
telescope was the only feature which seemed to interest him, and 
his plan was to devote nearly all the funds to making the largest 
lens possible. He did not see why such a complicated instrument 
as that used by astronomers was necessary. The troublesome prob- 
lem of seeing a heavenly body through a telescope had to be ex- 
plained to him. 



56 ASTRONOMICAL INSTRUMENTS 

But let us try the experiment of pointing a great tele- 
scope at a star. A result which perhaps we have not 
thought of would be immediately presented to our sight. 
The star, instead of remaining in the field of view* of 
the telescope, very soon passes out of it by the diurnal 
motion. This is because, as the earth revolves on its axis, 
the star seems to move in the opposite direction. This mo- 
tion is multiplied as many times as the telescope magni- 
fies. With a high power, the star is out of the field 
before we have time to examine it. 

Then it must also be remembered that the field of view 
is also magnified in the same way, so that it is smaller 
than it appears, in proportion to the magnifying power. 
For example, if a magnification of one thousand be used, 
the field of view of an ordinary telescope would be about 
two minutes in angular measure, a patch of the sky so 
small that to the naked eye it would look like a mere 
point. It would be as if we were looking at a star 
through a hole one eighth of an inch in diameter in the 
roof of a house eighteen feet high. If we imagine our- 
selves looking through such a hole and trying to see a 
star we shall readily realise how difficult will be the 
problem of finding it and of following it in its motion. 

This difficulty is overcome by a suitable mounting of 
the telescope, so as to turn on two axes, at right angles to 
each other. By the mounting is meant the whole system 
of machinery by the aid of which a telescope is pointed 



*By this term is meant the small circular patch of the sky 
which we see by looking into the telescope. 



MOUNTING OF THE TELESCOPE 57 

at a star and made to follow it in its diurnal motion. In 
order not to distract the attention of the reader by be- 
ginning a study of the instrument with a view of all the 
details, we first give an outline, showing the relation of 
the axes on which the telescope turns. The principal 
axis, called the polar axis, is adjusted so as to be parallel 
to the axis of the earth, and therefore to point at the 
celestial pole. Then, as the earth turns from west to- 
ward east, a clockwork connected with this axis turns the 




Fig. 11. — Axes on which a Telescoj^e hiviis. 

instrument from east toward west, with an equal motion. 
Thus the rotation of the earth is neutralized, as it were, 
by the corresponding rotation of the telescope in the 
opposite direction. When the instrument is pointed at 
a star and the clockwork set going, the star when once 
found will remain in the field of view. 

In order that a telescope may be directed at any point 
of the heavens at pleasure, there must be another axis, 
at right angles to the polar axis. This is called the 



58 ASTRONOMICAL INSTRUMENTS 

declination axis. It passes through a sheath fixed to the 
upper end of the polar axis so as to form a cross like 
the letter T. Bj turning the telescope on the two axes, 
it can be pointed wherever we choose. 

Owing to the polar axis being parallel to that of the 
earth, its inclination to the horizon is equal to the lati- 
tude of the place. In our latitudes, especially in the 
southern portions of the United States, it will be nearer 
horizontal than vertical. But in the observatories of 
northern Europe, it is more nearly vertical. 

It will be seen that the contrivance we have described 
does not solve the problem of bringing a star into the 
field of view of the telescope, or as we commonly say, 
of finding it. We might grope round for minutes or 
even hours without succeeding in this. There are two 
processes by which a star may be found : 

Every telescope for astronomical purposes is supplied 
with a smaller telescope fastened to the lower end of its 
tube, and called the finder. This finder is of low magni- 
fying power, and therefore has a large field of view. 
By sighting along the outside of it, the observer, if he 
can see the star, can point the finder at it so nearly that 
it wdll be in the field of view of the latter. Having found 
it there, he moves the telescope so that the object shall 
be seen in the centre of the field. Having brought it 
there, it is in the field of view of the main telescope. 

But most of the objects which the astronomer has to 
observe are totally invisible to the naked eye. He must, 
therefore, have a system by which a telescope can be 
pointed at a star, without any attempt on his part to see 



THE MAKLXG OF TELESCOPES 59 

the latter. This is clone bj graduated circles, one of 
which is attached to each axis. One of these circles has 
degrees and fractions of a degree marked upon it, so as 
to show the declination of that point in the heavens at 
which the telescope is pointed. The other, attached to 
the polar axis, and called the hour circle, is divided into 
twenty-four hours, and these again into sixty minutes 
each. When the astronomer wishes to find a star, he 
simply looks at the sidereal clock, subtracts the right 
ascension of the star from the sidereal time," and thus 
gets its "hour angle" at the moment, or its distance east 
or west of the meridian. He sets the declination circle 
at the declination of the star, that is, he turns the tele- 
scope until the degree on the circle seen through a mag- 
nifying aparatus is equal to the declination of the star; 
and then he turns the instrument on the polar axis until 
the hour circle reads its hour angle. Then, starting his 
clockwork, he has only to look into the telescope and 
there is the object. 

If all this seems a complicated operation to the reader, 
he has only to visit an observatory and see how simply it 
is all done. He may thus in a few minutes gain a practi- 
cal idea of sidereal time, hour angle, declination, etc., 
which will make the whole subject much clearer than any 
mere description. 

The Making of Telescopes 

Let us return to some interesting matters, mostly his- 
torical, connected with the making of telescopes. The 
great difficulty, which requires special native skill of the 



60 ASTRONOMICAL INSTRUMENTS 

rarest kind, is, as we have already intimated, that of con- 
structing the object-glass. The slightest deviation from 
the proper form — a defect consisting in some part of 
the object-glass being too thin by a hundred thousandth 
part of an inch — would spoil the image. 

The skill of the optician who figures the glass, that is 
to say, who polishes it into the proper shape, is by no 
means all that is required. The making of large disks 
of glass of the necessary uniformity and purity is a 
practical problem of equal difficulty. Any deviation from 
perfect uniformity in the glass will be as injurious to its 
performance as a defect in its figure.* 

A century ago it was found especially difficult to make 
flint glass of the necessary uniformity. This substance 
contains a considerable amount of lead, which, during 
the process of melting the glass, would sink toward the 
bottom of the pot, thus making the bottom portion of 
greater refracting power than the upper portion. The 
result was that, at that time, a telescope of four or five 
inches aperture was considered of great size. Quite early 
in the centur}^, Guinand, a Swiss, found a process by 
which larger disks of flint glass could be made. He pro- 
fessed to have some secret process of doing this, but there 
is some reason to believe that his secret consisted only in 
the constant and vigorous stirring of the melted glass 

* It is frequently proposed by persons not acquainted with the 
delicate points of the problem to make a telescope of large size by 
putting together different pieces of glass, each of the proper shape, 
to form a lens. The idea, ingenious though it looks, is thor- 
oughly impracticable, for the simple reason that it is impossible to 
make two pieces of glass of exactly the same refracting power. 



ALVAN CLARK AND HIS GENIUS 61 

while it was being fused in the pot. However tliis may 
have been, he succeeded in making disks of larger and 
larger size. 

To utilize these disks required an optician of corre- 
sponding skill to grind and polish them into proper 
shape. Such an artist was found in the person of Fraun- 
hofer, of Munich, who, about 1820, made telescopes as 
large as nine inches aperture. He did not stop here, but, 
about 1840, succeeded in making two objectives, each of 
fourteen German inches, or about fifteen English inches 
in diameter. These, far exceeding any before made, were 
at the time regarded as marvellous. One of these instru- 
ments was acquired by the Pulkova Observatory in Rus- 
sia ; the other was acquired by the Harvard Observatorj^ 
at Cambridge, ]Mass. The latter, after a lapse of more 
than half a century, is still in efficient use. 

Alvan Clark and His Genius 

After Fraunhofer's death it was doubtful whether his 
skill had died with him, or had passed to a successor. 
The latter appeared where none would have thought of 
looking for him, in the person of an obscure portrait 
painter of Cambridgeport, Mass., named Alvan Clark. 
The fact that such a man, with scarcely the elements 
of technical education and without training in the use 
of optical instruments, should have done what he did, 
illustrates in a striking way what an important element 
native talent is in such a case. He seemed to have an 
intuitive conception of the nature of the problem, 
coupled with extraordinary acuteness of vision in solving 



62 ASTRONOMICAL INSTRUMENTS 

it. Moved by that irrepressible impulse which is a mark 
of genius, he purchased in Europe the rough disks of 
optical glass necessary to make small telescopes. Having 
succeeded in making one of four inches aperture to his 
satisfaction, the problem was to make his skill known to 
astronomers. I regret to say that he found this a very 
difficult part of his task. The director of the Harvard 
Observatory would not believe that Mr. Clark could make 
a realty good telescope. When the optician took his 
first instrument up to the observatory to be tested, the 
astronomer called his attention to the fact that it showed 
a little tail attached to the star, which, of course, had no 
real existence, and was supposed to arise from a serious 
defect in the figure of the glass. Mr. Clark saw it, but 
was sure it had not been there before. He could not ex- 
plain it at the time, but afterwards found that it was 
caused by the unequal temperature of the air in the tube 
of the telescope when it was exposed under the sky at 
night. 

Unable to secure any effective recognition at home, 
he determined to try abroad. He made a larger instru- 
ment, scanned the heavens with it and discovered several 
close and difficult double stars. He wrote out descrip- 
tions of these objects and sent them to Rev. W. R. 
Dawes, an amateur astronomer in England, devoted to 
this branch of the science. Mr. Dawes was a lovely char- 
acter. He looked at the objects described by Clark and 
found great difficulty in making them out. Yet the de- 
scriptions were so accurate that it was evident to him 
that Mr. Clark's instrument must be of the highest class. 



CLARK'S GREAT TELESCOPES 63 

He wrote asking him to look at some other objects and 
describe them. When the description was received it was 
found to be exact. No doubt could remain. The result 
was a further correspondence, the purchase by Mr. 
Dawes of the largest and best instrument that ^Ir. Clark 
could then make, and a friendship which continued as 
long as Mr. Dawes lived. 

Mr. Clark now secured recognition in his own country 
and became ambitious to make the largest refracting 
telescope that had ever been known. This was one of 
eighteen inches diameter, which was completed about 
I860 for the University of Mississippi. While testing 
it at his workshop, a discovery of a most interesting 
character was made wdth it by Mr. George B. Clark, the 
son. This was a companion of Sirius, which had been 
known to exist by its attraction on Sirius, but had never 
been seen by human eye. The breaking out of the Civil 
War prevented the University of Mississippi from tak- 
ing the telescope, and the latter was acquired by citizens 
of Chicago. It is now mounted at the Northwestern 
University in Evanston, 111. 

The making of disks of glass of larger and larger 
size was continued by the great glass works of Chance 
& Company, in England. But they found the work 
too delicate and too troublesome, and allowed it to pass 
into the hands of Feil of Paris, son-in-law of Guinand. 
With the glass supplied by these two parties, Mr. Clark 
made larger and larger telescopes. First was the twenty- 
six-inch telescope for the N.aval Observatory at Wash- 
ington and a similar one for the University of Virginia. 



64 ASTRONOMICAL INSTRUMENTS 

Then followed a still larger instrument, thirty inches in 
diameter, for the Observatory of Pulkova, Russia. Next 
was cov>^pleted the thirty-six-inch instrument of the Lick 
Observatory, which has done such splendid work. 

After the death of Feil, the business was taken up by 
Mantois, who made optical glass of a purity and uni- 
formity that no one before him had ever approached. 
He furnished the disks with wliich the Clarks figured 
the objective for the Yerkes telescope of the University 
of Chicago. This is about forty inches in diameter, and 
is the largest refracting telescope now in actual use for 
astronomlcpT purposes. 

Our readers have doubtless been interested in the great 
telescope of the Paris Exposition of 1900, which is yet 
larger than that of Chicago, being of forty-seven inches 
aperture. Tliis instrument is of such immense size that 
it cannot be mounted and pointed at the heavens in the 
usual way. It is therefore fixed in a horizontal, north 
and south position, and the rays of the object to be ob- 
served are reflected into it by an immense plane mirror. 
The question whether this contrivance has been success- 
ful with so large an instrument is one that is not yet 
settled with astronomical precision. Nothing has yet 
been done with this instrument, which, it is feared, is 
so imperfect in maxe as to serve no better purpose than 
that of a toy. 

The engineering problem of mounting a great tele- 
scope is by no means a simple one. It was one in which 
Mr. Clark was less successful than in the construction of 
his object-glasses. In the case of the later telescopes the 



nil 



GREAT TELESCOPES 



65 




Fig. 12. — Great Telescope of the Yerkes Observatory, mounted by Warner & 

Swazey. 



66 ASTRONOMICAL INSTRUMENTS 

mountings of the great instruments were made by other 
parties. That of the Pulkova telescope was made by the 
Repsolds of Hamburg, the most noted makers of fine 
astronomical instruments in Europe. The Lick and 
Chicago telescopes were mounted by Warner & Swazey, 
of Cleveland, Ohio, who are gaining the highest reputa- 
tion in this class of work. In the case of the Chicago 
telescope, arrangements were devised by them which sur- 
pass all ever before thought of. The observer has only 
to touch electric buttons to have all the work of pointing 
and moving the telescope performed by electricity. 



II 

The Reflecting Tei^escope 

Although the refracting telescope is that in most 
general use, there is another form of instrument of radi-' 
cally different construction. Its main feature is that 
the functions of the object-glass are performed by a 
slightly concave mirror. That such a mirror reflects 
parallel rays falling upon it to a focus, is doubtless well 
known to our readers. The focus is situated about half- 
way between the mirror and its centre of curvature. 

This form of instrument has an enormous advantage 
in its freedom from the "secondary aberration" which 
we have already described as inherent in the refracting 
telescope. Another advantage which it possesses is that 
it can be made of larger dimensions than the other. The 
extreme limit so far reached in the refractor, as we have 
already stated, is four feet; but the forty-inch aperture 
of the Yerkes telescope is, up to the present time, the 
limit in actual use for astronomical research. But, more 
than half a century ago. Lord Rosse constructed his 
great reflector of six feet diameter. Judging by its 
size alone, this instrument ought to give several times 
more light, and therefore show far minuter stars, than 
any refracting telescope yet made. But, for some rea- 
son, its performance — and, indeed, that of reflectors 
generally — has not corresponded to the size. 



68 ASTRONOMICAL INSTRUMENTS 

The practical difficulties in using a reflector are several 
in number. The first and most obvious one is that the 
rays are reflected back in the direction from which they 
came. To see the image the observer must look into the 
mirror as it were. If he does this directly, his head and 
shoulders will cut off' the hght that falls on at least the 
central regions of the mirror. Some contrivance for re- 
flecting this light away is therefore necessary. Two 
ways of doing tliis are in use. In what is known as the 
Cassegranian reflector, a smaller, slightly convex mirror 
is interposed between the focus and the principal mirror. 
An opening is made in the centre of the latter, through 
which the rays are reflected back by the smaller mirror. 
The curvature and positions of the two are so adjusted 
that the image of the distant object shall be formed in 
this opening. The only telescope of this kind in actual 
use is the great Melbourne reflector, of four feet diam- 
eter, made by Sir Howard Grubb, of Dublin. 

The contrivance most in use was designed by Sir Isaac 
Ne^vton. It consists of a diagonal reflector, which may 
be a mere glass prism, placed just inside the focus. Its 
reflecting surface makes an angle of forty-five degrees 
with the axis of the telescope, and therefore reflects the 
rays laterally to the side of the tube. Here they are 
observed with an ordinary eyepiece. This instrument is 
known as the Newtonian reflector. 

It is remarkable that, notwithstanding the immense 
improvement in the mechanical processes necessary in 
constructing and mounting a reflecting telescope, no at- 
tempt has ever been made to even equal Lord Rosse's 




Fig. 13. — Section of a Newtonian Reflecting Telescope. 



70 ASTRONOMICAL INSTRUMENTS 

great instrument in dimensions. The largest mirrors 
so far successfully made and used have been about four 
feet in diameter. About fifty years ago, Mr. Lassell 
made one of this size, with which he discovered two 
new satellites of Uranus. More recently, Mr. A. A. 
Common, F.R.S., has constructed a mirror of the same 
size. This has been used in taking photographs of 
nebulae and other faint objects, for which this form of 
telescope seems well designed. 

The great difficulty in using a large mirror is that it 
bends under the influence of its own weight. It would 
seem that when the diameter exceeds four feet, no way 
of completely avoiding this difficulty has yet been put 
into successful use. A mirror of five feet diameter is, 
however, being made at the Yerkes Observatory by Mr. 
Ritchie, in which, it is hoped, all the difficulties will be 
surmounted. 

In the instruments of Lord Rosse and Mr. Lassell, the 
mirror was made of an alloy, known as speculum metal. 
Recently, however, the use of speculum metal has been 
superseded by another arrangement. The concave mir- 
ror is made of a large disk of glass, which is ground and 
polished into nearly spherical form, or to speak more ac- 
curately, a parabolic form, because the latter is necessary 
to bring all the rays to one focus. A thin coating of 
silver is then deposited on the surface of the glass, which 
is susceptible of a high polish, and reflects more light 
than polished metal. 



Ill 

The Photogkaphic Telescope 

One of the greatest advances in practical astronomy 
in our time has been brought about by photographing 
the heavenly bodies. This is so simple a process that the 
slowness of its introduction may seem curious. Back in 
the early '40's, Professor Draper, of New York, the well- 
known chemist, succeeded in making a daguerreotype 
of the moon. When the system of photography by our 
present process on a glass negative was invented. Pro- 
fessor Bond, of the Harvard Observatory, and Mr. L. M. 
Ilutherfurd, an eminent astronomer of New York, both 
began to apply the art to the moon and stars. Mr. 
Rutherf urd brought his work to such perfection that his 
photographs of the Pleiades and other clusters of stars 
are still of great value in astronomy. 

A photograph of the stars can be made by an ordinary 
camera if we only mount it like an equatorial telescope 
so that it shall follow the star in its diurnal motion. A 
very few minutes exposure will suffice to take a picture 
of more stars than can be seen by the naked eye ; in fact, 
with a large camera, this will not require a minute. But 
what is generally used by the astronomer is a photo- 
graphic telescope. Any ordinary telescope will serve 
the purpose, but in order to get the best results the 
object-glass of the telescope must be especially made to 



72 ASTRONOMICAL INSTRUMENTS 

bring to a focus those rays of light to which the photo- 
graphic film is most sensitive. So rapid has been the 
progress during the past few years that the greater part 
of the astronomical work of the future seems likely to be 
done by photography. The great advantage of the 
method is that when a picture either of some heavenly 
body or of the stars in the sky is taken, it can be studied 
and measured at leisure with all the care the astronomer 
chooses to bestow upon it, while the observation in the 
heavens is nearly always more or less hurried, and made 
difficult by the diurnal motion of the star. 

Formerly the spots on the sun were investigated by 
watching that luminary through the telescope, recording 
the number of spots, and measuring their position on the 
solar disk. Now, at tlie Greenwich Observatory and else- 
where, a photograph of the sun is taken almost every 
day, and the position of the spots is found by measuring 
the photograph. Thus a study of the sun and the 
changes going on on its surface is kept up from year to 
year. 

Formerly the astronomer studied the physical con- 
stitution of a comet by making a drawing of it. This 
was a rather uncertain process, and as a general rule no 
two men would quite agree in the minute details. Now 
the comet is photographed and a study is made upon the 
negative. The same remark applies to nebulae. Draw- 
ings of them are no longer made — only photographs 
which show a great deal more than any drawing will. 



IV 

The Spectroscope 

The spectroscope is an instrument for analysing 
light. It is a much more recent instrument than the 
telescope, having first been applied to astronomical ob- 
servation about 1864o To convey an intelligent idea 
of its use we must say something about the heat and light 
radiated by the heavenly bodies. 

We know that the sun, a gas light, or other bright 
body gives us heat as well as light. A very simple obser- 
vation will show that the rays of heat proceed in straight 
lines like those of light, and that they can pass through 
air and other transparent bodies without warming them, 
just as light does. If we make a large fire on the hearth 
in a perfectly cold room, we shall feel the heat on our 
faces although the air may be frosty. A striking experi- 
ment is that of making a lens out of ice and using it as 
a burning glass. The rays of the sun passing through 
the ice may be concentrated so as to burn the hand, and 
that without the ice melting. 

It was formerly supposed that heat and light were two 
distinct agents ; now it is known that such is not the case. 
As emitted by a hot body both may be called by the 
general name of radiance. All radiance, when it falls 
on a surface, produces heat, just as the blaze of the fire 
produces heat on the walls of a room. But not all radi- 



74 ASTRONOMICAL INSTRUMENTS 

ance affects the optic nerve of the eye so as to produce a 
sensation of light and enable us to see bodies. 

It is now know that radiance consists of something in 
the nature of waves in an ethereal medium which fills all 
space, even to the most distant star. These waves are 
exceedingly short. To form an idea of their length we 
must measure by the micron, which is one thousandth 
of a millimetre. Those which produce the sensation of 
light on the optic nerve mostly range between four and 
seven tenths of a micron. This allows between forty and 

eighty thousand waves 

•"T'^^.^^^^'^'T^^''^^— '■''^"^ to th^ inch. We rep- 

: • I resent these waves by 

Fig. 14.— Wave Length of Light. the little wave line in 

the figure. The dis- 
tance between the dotted lines is the wave lengths. The 
peculiar feature of the radiance emitted by the sun, or 
any other body that is not transparent, is that it is not 
all of the same wave length, but of a very wide range 
of wave lengths all mixed together. We must imagine 
that between the rays which we represent in the figure 
there are an infinity of others, all varying in their wave 
lengths. In this respect radiance is like the waves of 
the ocean, which range in length from several hundred 
yards to a few inches, all piled upon each other. 

When the radiance passes through a glass prism it is 
refracted from its course. Different wave lengths are 
refracted differently, but waves of the same length are 
always refracted by the same amount. This is shown by 
the familiar experiment of forming a spectrum of the 



THE SPECTROSCOPE 



75 



sun with a triangular prism. Arranging the light to 
be thrown on a screen, we see red light at the bottom, 
then yellow above it, then in succession, green, blue, and 
violet. This arrangement of colours 
on a surface is called a spectrum. The 
colour of the light in the spectrum 
depends on the wave length. If the 
wave length is greater than about 
seventy-five one-hundredths of a mi- 
cron, that is, one forty-four-thou- 
sandth of an inch, the eye does not 
see it, and, for us, it passes simply as 
heat. From this length to one fifty- 
thousandth it looks red, when a little 
shorter it looks scarlet, then yellow, 
and so on. Shorter than forty-three 
one-hundredths of a micron it is diffi- 
cult to see it at all. But the violet 
light affects the photographic plate 
even more strongly than the light 
which looks brightest to the eye. The 
light which is most easily photo- 
graphed is the blue and violet, and as 
we go toward the red the photo- 
graphic effect diminishes. 

All bodies emit radiance, but, at 
ordinary temperatures, the wave 
lengths of this radiance are too long to be visible 
to the eye. Not until we heat a body red hot 
does it emit radiance of wave length short enough to 



: 

L 

K 
H 



F 
b 

D 

c 

B 
A 




VIOLET 








^^^ 
















INDICO 

BLUE 
GREEN 












' 




ORANGE 
RED 











Fig. 15. — Arrange- 
ment of the Coloura 
of the Spectrum), 
with the Dark Lhtes 
A,B, C.D.etc, of 
the Spect7'um. 



76 ASTRONOMICAL INSTRUMENTS 

form light. As we make it hotter it still emits more and 
more waves of long wave lengths, and also waves of 
shorter and shorter wave lengths. Thus as we heat up 
a piece of iron, it appears first as red hot, and afterward 
as white hot. 

The possibility of reaching conclusions about the con- 
stitution of a hot body from the light which it emits arises 
from the fact that different bodies emit light of different 
wave lengths. If the body is solid, it emits light of all 
wave lengths, and we cannot tell much about it. But if 
it is a mass of transparent gas, it only emits light of cer- 
tain wave lengths, depending on the nature of the gas. 

The easiest way of making a gas emit its peculiar 
light is by passing an electric spark or current through 
it. Then, if we analyse the light produced by the spark 
with a prism, we find that the spectrum is composed of 
one or more bright lines, varying in position according 
to the nature of the gas. Thus we have a spectrum of 
hydrogen, another of oxygen, and others of almost all 
the bodies which we know. Solid bodies, including all 
the metals, can be made to give their spectrum by being 
heated so intensely by the electric spark that a small 
quantity of the body is changed into a gas. Thus we 
may even form a spectrum of iron, which the practised 
observer can immediately detect as iron by the position 
and arrangement of the lines of the spectrum. 

How the Stars are Analysed 

The fundamental principle of spectrum analysis is 
that if the light of an incandescent body passes through 



HOW THE STARS ARE ANALYSED 77 

a gas which is cooler than the body, the latter will cull 
out and absorb from the light those wave lengths which 
it would emit if it were itself incandescent. The result 
is that the spectrum from the solid body w^ill be seen 
crossed by certain dark lines, depending on the nature 
of the gas through which the light has passed. Thus, 
if we observe an electric light through a prism in its 
immediate neighbourhood, the spectrum will be unbroken 
from one end to the other. But if the light is at a great 
distance, we shall see it crossed by a great number of dark 
lines. These lines are produced by the air through which 
the light has passed culling out the light which has cer- 
tain wave lengths. It is of interest that the aqueous va- 
pour in the air is the most powerful agent in this, and 
culls out great groups of lines, by which its presence in 
the air can be immediately detected. The darkest of the 
lines found in the spectrum of the sun are designated by 
the letters A, B, C, etc., as shown in the preceding 
figure. 

We may describe the spectroscope in the most compre- 
hensive way b}^ saying that it is an instrument for 
studying the spectra of bodies, whether in the heavens 
or on the earth. 

The studies of the heavenly bodies with the spectro- 
scope have two objects. One is to determine the nature 
of the bodies ; the other their motions to or from us. 
The possibility of the latter is one of the most wonderful 
achievements of modern science. If a star is coming 
toward us, the wave length of the light which it emits is 
slightly shorter in consequence of the motion ; if it is 



78 ASTRONOMICAL INSTRUMENTS 

going away from us, it is longer. Thus, by measuring 
the positions of its lines in the spectrum, it is possible 
to determine whether a star is approaching us or 
moving away from us. 

In recent years the studies of the spectra of stars have 
been made almost entirely by photography. It is found 
that, as in other cases, the sensitive plates now used in 
that art will take impressions of objects which the eye 
cannot see in the telescope. So the astronomer photo- 
graphs the spectrum of a star, which will show all the 
lines he can see with the naked eye, and perhaps a great 
many more. The positions of these lines are measured 
and studied, and the astronomer's conclusions are drawn 
from these studies. 



V 

Other Astronomical Instruments 

It is commonly supposed that the principal work of 
an astronomer is to study the stars as he sees them in 
his telescope. This is true only in the sense that a tele- 
scope is a necessary part of ahuost every astronomical 
instrument. But the mere studying of a star with a 
telescope is a very small part of the astronomer's work. 
The most important practical use of astronomy to our 
race consists in the determination of the latitudes and 
longitudes of points on the earth's surface, so that we 
maj^ know where towns and cities are situated and be 
able to m.ake a map of a state or country. This re- 
quires a knowledge of the exact positions of the stars in 
the heavens, that is to say, of their right ascension and 
declination. We have shown in a former chapter how 
these quantities correspond to longitude and latitude on 
the earth's surface. Through that correspondence an 
observer may determine his latitude by the star's dec- 
lination and his longitude by its right ascension, com- 
bined with a knowledge of the sidereal time at a place 
of known longitude. 

The figures and dimensions of the planets, the motions 
of the satellites, the orbits of planets and comets, the 
structure of nebulae and clusters of stars — all these offer 
fields of astronomical investigation to which there is 



80 



ASTRONOMICAL INSTRUMENTS 



no end, and in order to make these investigations other 
instruments besides the telescope are necessary. 

The Meridian Circle and Clock 

The problem which demands most attention from the 
working astronomer in an observatory is the determina- 
tion of the positions of the heavenly bodies. The prin- 




FiG. 16. — A 2Ierklian Instrument. 

cipal instrument for making these determinations is the 
w.eridian circle, called also a meridian instrument. This 
consists of a telescope supported on a horizontal east and 
west axis, at right angles to its length, so that its line 
of sight can move only along the meridian. If it points 



MERIDIAN CIRCLE AND CLOCK 81 

exactly south you can turn it on the axis until the line 
of sight passes tlu'ough the zenith, and still farther until 
it passes through the pole on the north horizon; but 
jou cannot turn it east or west. This might seem to re- 
strict its usefulness, but it is on tliis restriction of its 
motion that its usefulness depends. The great value 
of this instrument is that it enables us to determine the 
right ascension of a star without taking any measure- 
ment but one of time. In a former chapter we described 
sidereal time, the units of which are slightly shorter 
than those of our ordinary time, so that a sidereal clock 
gains about two hours every month on an ordinary clock. 
The sidereal time at which a star crosses the meridian is 
the same as its right ascension ; the problem of determin- 
ing the latter, therefore, is the simplest in the world. 
We start our sidereal clock, set it on the exact sidereal 
time, point the telescope of the meridian circle to various 
stars as they are about to cross the meridian, and note 
the exact moment at which each star passes. In the 
instrument the meridian is shown by a very fine fibre or 
spider's web fixed in the focus of the telescope. The 
moment when the image of the star as seen in the tele- 
scope crosses this spider line is that of passing the me- 
ridian. The time by the sidereal clock then shows the 
star's right ascension. If the clock could be set with 
perfect exactness and the instrument revolved exactly 
in the plane of the meridian, right ascensions would be 
determined in the very simple way we have described. 

It unfortunately happens, however, that no clock can 
be set with such exactness as to satisfy the requirements 



82 ASTRONOMICAL INSTRUMENTS 

of the astronomer, who wants to know the time down to 
the tenth or even to the hundredth of a second. More- 
over, no meridian circle can have its axis set so exactly 
east and west that the instrument shall not deviate a little 
from the meridian. The astronomer must therefore make 
allowances for the error of his clock and for the deviation 
of his instrument; and these require much careful ob- 
servation and calculation. Even when he does the best 
he can, a single observation will always be liable to little 
errors which he wishes to make as small as possible. He 
does this by repeatedly determining the position of every 
star which he puts upon his list. He generally has to be 
satisfied with three or four observations on the great 
mass of the stars, but on the more important stars he 
makes them by scores or hundreds. 

To determine the declination of a star, a graduated 
circle is necessary. This consists of a brass or steel circle, 
much like a carriage wheel, of which the axis is the same 
as that on which the telescope of the meridian instrument 
turns. The circle is firmly attached to the axis so 
that it must turn with the telescope as the latter sweeps 
along the celestial meridian. The graduations of the 
circle consist of very fine marks or lines all round its 
circumference. The latter being divided into three hun- 
dred and sixty degrees, every degree is marked by such 
a line. Between these it is common to mark thirty inter- 
mediate lines, which are therefore two minutes apart. 
Attached to one or both the stone piers which support 
the instrument are four microscopes, so fixed that the 
graduations on the circle are seen through them. When 



MERIDIAN CIRCLE AND CLOCK 83 

the instrument is turned on its axis, all these graduations 
pass successively under each microscope, so that they 
can be seen by the observer looking through the latter. 
The position of the star is determined b}^ measures with 
the microscope on the graduation which happens to be 
under it when the telescope is pointed at a star. 

The equatorial telescope and the meridian circle are 
the two principal instruments in the astronomical outfit 
of an observatory. Many other instruments are more 
or less in use for special purposes, but they are not of 
great interest, save to one who is making a special study 
of astronomy and who must therefore refer to books 
specially written for the professional student of the 
subject. 

The precision with which a practised observer can 
note the time of transit of a star over the thread of his 
instrument is remarkable. One method of doing this 
consists in listening to and counting the beats of the 
clock as the star approaches and crosses the thread. 
He watches the exact position of the star at the beat 
before the transit, and again at the beat following. By 
comparing in his mind the opposite distances of the 
star from the thread at the two clock beats, he estimates 
the number of tenths of the second at which the transit 
took place, and records the time in his notebook. 

This method is now superseded in most observatories 
by that of registration on a chronograph. This instru- 
ment consists of a revolving cylinder, covered with 
paper, having a pen-point resting upon it, so that, as 
the cylinder revolves, the pen leaves a trace on the paper. 



84 ASTRONOMICAL INSTRUMENTS 

The pen is so connected with an electric current passing 
tlirough the clock, and through a key held by the ob- 
server, that every beat of the clock and every pressure 
of the key by the observer makes a notch in the trace 
left by the pen. When the observer sees that a star has 
reached the thread of his instrument he presses the key, 
and the position of the notch thus made in the pen-trace 
between two notches made by the clock gives the moment 
at which the key was pressed. 

The astronomer's clock must be of the highest at- 
tainable perfection, running for a whole day or more 
without a deviation of one tenth of a second. With a 
common house clock, the change in the length of the 
pendulum produced by changes of temperature between 
the day and night would cause deviations of several 
seconds. Hence in the astronomical clock these changes 
must be neutralised. This is done by making the pen- 
dulum of such a combination of different materials that 
the unequal expansions of the latter shall neutralise each 
other. The most common combination is that of a steel 
rod bearing at its lower end a steel or glass jar of 
quicksilver, which serves as the bob of the pendulum. 
Then, when the temperature rises, the upward expansion 
of the quicksilver compensates the downward expansion 
of the steel. 



PART III 
THE SUN, EARTH, AND MOON 



I 

An Introductory Glance at the Solar System 

We have shown how this comparatively small family 
of bodies, on one of which we dwell, forms as it were a 
little colony by itself. Small though it be when com- 
pared with the whole universe as a standard, it is for us 
the most important part of the universe. Before pro- 
ceeding to a description of its various bodies in detail 
we must take a general view to show of what kind of 
bodies it is formed and how it is made up. 

First of all we have the sun, the great shining central 
body, shedding warmth and light on all the others and 
keeping the whole system together by virtue of its 
powerful attraction. 

Next we have the planets, which revolve round the sun 
in their regular orbits, and of which our earth is one. 
The word planet means wanderer, a term applied in 
ancient times because these bodies, instead of keeping 
their places among the fixed stars, seemed to wander 
about among them. The planets are divided into two 
quite distinct classes, termed major and minor. 

The major planets are eight in number and are, next 
to the sun, the largest bodies of the system. For the 
most part their distances from the sun are arranged in a 
close approach to a certain regular order, ranging from 
nearly forty millions of miles in the case of Mercury, 



88 THE SUN, EARTH, AND MOON 

the nearest one, to three thousand milHons in the case of 
Neptune. The latter is therefore seventy times as far 
from the sun as Mercury. Still wider is the range of 
their times of revolution. Mercury performs its circuit 
round the sun in less than three of our months — Neptune 
takes more than one hundred and sixty years for his long 
journey. It has not yet made half a revolution since its 
discovery in 1846. 

The major planets are separated into two groups of 
four planets each, wdth quite a broad gap between the 
groups. The inner group is composed of much sm.aller 
planets than the outer one ; all four together would not 
make a body one quarter the size of the smallest of the 
outer group. 

In the gap between the two groups revolve the minor 
planets, or asteroids as they are commonly called. They 
are very small as compared with the major planets. So 
far as we know they are all situated in a quite wide belt 
ranging between a little more than the distance of the 
earth out to four times that distance. For the most 
part they are about three or four times as far from the 
sun as the earth is. They are also distinguished from 
the major planets by their indefinite number; some five 
hundred are now known, and new discoveries are con- 
tinually being made at such a rate that no one can set 
any exact limit to them. 

A third class of bodies in the solar system comprises 
the satellites, or moons. Several of the major planets 
have one or more of these small bodies revolving round 
them, and therefore accompanying them in their revolu- 



GLANCE AT THE SOLAR SYSTEM 89 

tion around the sun. The two innermost planets, Mer- 
cury and Venus, have no satelHtes, so far as we yet 
know. In the case of the other planets their number 
ranges from one (our moon) to eight, which form the 
retinue of the planet Saturn. Each major planet, Mer- 
cury, and Venus excepted, is therefore the centre of a 
system bearing a certain resemblance to the solar system. 
These systems are sometimes designated by names de- 
rived from those of their central bodies. Thus we have 
the Martian System, composed of Mars and its satellites ; 
the Jovian System, composed of Jupiter and its five 
satellites; the Saturnian System^ comprising the planet 
Saturn, its rings, and sacellites. 

A fourth class of bodies consists of the comets. These 
move round the sun in very eccentric orbits. We see 
them only on their approach to the sun, which, in the 
case of most of these bodies, occurs only at intervals of 
centuries, or even thousands of years. Even then a 
comet may fail to be seen unless under favourable 
conditions. 

Besides the preceding bodies we have a countless num- 
ber of meteoric particles revolving round the sun in 
regular orbits. These are probably related in some way 
to the comets. They are completely invisible except as 
they strike our atmosphere, when we see them as 
shooting stars. 

The following is the arrangement of the planets in 
the order of their distance from the sun and with the 
number of satellites of each : 



90 THE SUN, EARTH, AND MOON 

/. Inner Group of Major Planets: 

Mercur3^ 

Venus. 

Earth, with one satellite. 

Mars, with two satellites. 

II. Group of Minor Planets, or Asteroids. 

III. Outer Group of Major Planets : 

Jupiter, with five satellites. 
Saturn, with eight satellites. 
Uranus, with four satellites. 
Neptune, with one satellite. 

Instead of taking up these bodies in the order of their 
distance from the sun, we shall, after describing the 
latter, pass over Mercurj^ and Venus to consider the earth 
and moon. Then we shall return to the other planets 
and describe them in order. 



II 

The Sun 

In a description of the solar system its great central 
body is naturally the first to claim our attention. We 
see that the sun is a shining globe. The first questions 
to present themselves to us are about the size and dis- 
tance of this globe. It is easy to state its size when we 
know its distance. We know by measurement, the angle 
subtended by the sun's diameter. If we draw two lines 
making this angle with each other, and continue them in- 
definitely through the celestial spaces, the diameter of 
the sun must be equal to the distance apart of the lines 
at the distance of the sun. The exact determination is 
a very simple problem of trigonometry. It will suffice 
at present to say that the measure of the apparent 
diameter of the sun, or the angle which it subtends to our 
eye, is thirty-two minutes, making this angle such that 
the distance of the sun is about 107.5 times its diameter 
in miles. If, then, we know the distance of the sun, we 
have only to divide it by 107.5 to get the sun's diameter. 

The various methods of determining the distance of 
the sun will be described in our chapter stating how dis- 
tances in the heavens are measured. The result of all 
the determinations is that the distance is very nearly 
ninety-three million miles, perhaps one or two hundred 
thousand miles more. Taking the round number, and 



9S THE SUN, EARTH, AND MOON 

dividing by 107.5, we find the diameter to be about 865,- 
000 miles. This is about one hundred and ten times the 
diameter of the earth. It follows that the volume or bulk 
of the sun is more than one million three hundred thou- 
sand times that of the earth. 

The sun's importance to us arises from its being our 
great source of heat and light. Were these withdrawn, 
not only would the world be enveloped in unending 
night, but, in the course of a short time, in eternal frost. 
We all know that during a clear night the surface of the 
earth grows colder through the radiation into space of 
the heat received from the sun during the day. With- 
out our daily supply, the loss of heat would go on until 
the cold around us would far exceed that which we now 
experience in the polar regions. Vegetation would be 
impossible. The oceans would freeze over, and all life 
on the earth would soon be extinct. 

The surface of the sun, which is all we can see of it, 
is called the photosphere. This term is used to distin- 
guish the visible surface from the vast invisible interior 
of the sun. To the naked eye, the photosphere looks 
entirely uniform. But through a telescope we see that 
the whole surface has a mottled appearance, which has 
been aptly compared to that of a plate of rice soup. 
Examination under the best conditions shows that this 
appearance is due to minute and very irregular grains 
which are scattered all over the photosphere. 

When we carefully compare the brightness of differ- 
ent regions of the photosphere, we find that the apparent 
centre of the disk is brighter than the edge. The differ- 



ROTATION OF THE SUN 93 

ence can be seen even without a telescope, if we look at 
the sun through a dark glass, or when it is setting in a 
dense haze. The falling off in the light is especially 
rapid as we approach the extreme edge of the disk, where 
it is little more than half as bright as at the centre. 
There is also a difference of colour, the light of the edge 
having a lurid appearance as compared with that of 
the centre. 

All this shows that the light of the sun is absorbed by 
an atmosphere surrounding the sun. We readily see 
that, the sun being a globe, the light which we receive 
from the edge of its disk leaves it obliquely, while that 
from the centre leaves it perpendicularly. The more 
obliquely the light comes from the surface, the greater 
the thickness of the sun's atmosphere through which it 
must pass, and hence the greater the portion lost by the 
absorption of that atmosphere. The sun's atmosphere, 
like our own, absorbs the green and blue rays more than 
the red. For this reason the light has a redder tint when 
it comes from near the edge of the disk. 

Rotation of the Sun 

Careful observations show that the sun, like the 
planets, rotates on an axis passing through its centre. 
Using the same terms as in the case of the earth, we call 
the points in which the axis intersects the surface the 
poles of the sun, and the circle around it halfway be- 
tween the poles the sun's equator. The period of rota- 
tion is about twenty-six days. As the distance around 
the sun is more than one hundred and ten times that 



94 THE SUN, EARTH, AND MOON 

round the earth, the speed of rotation must be more than 
four times that of the earth's rotation to make it com- 
plete the circuit in the time that it does. At the sun's 
equator the speed is more than a mile a second. 

The most curious feature of this rotation is that it 
is completed in less time at the equator than at a distance 
on each side of the equator. Were the sun a solid body, 
like the earth, all its parts would have to rotate at the 
same time. Hence the sun is not a solid bod}'^, but must 
be either liquid or gaseous, at least at its surface. 

The equator of the sun is inclined six degrees to the 
plane of the earth's orbit. Its direction is such that in 
our spring months the north pole is turned six degrees 
away from us and the central point of the apparent disk 
is about that amount south of the sun's equator. In our 
summer and autumn months this is reversed. 

The Sun's Density and Gravity 

By the mean density of the sun we refer to the average 
specific gravity of the matter composing it, or the ratio 
of its weight to that of an equal volume of water. It is 
known that the density is only about one fourth that of 
the earth, and about four tenths greater than that of 
water. Stated with more exactness, the figures are : 

Density of sun : Density of earth = 0.2554. 

Density of sun: Density of water =: 1.4115. 

The mass or weight of the sun is about 334,000 times 
that of the earth. 

The force of gravity at the sun's surface is 27 times 
that of the earth. If it were possible for a human being 



SPOTS ON THE SUN 95 

to be placed there, an ordinary man would weigh two 
tons, and be crushed by his own weight. 

Spots on the Sun 

When the sun is carefully examined with a telescope, 
one or more seemingly dark spots will generally, though 
not always, be seen on its surface. These are, of course, 
carried around by the rotation of the sun, and it is by 
means of them that the time of rotation is most easily 
determined. If a spot appears at the centre of the disk 
it will, in six da^^s, be carried to the western edge, and 
there disappear. At the end of about two weeks it will 
reappear at the eastern edge unless it has, in the mean- 
time, died away, which is frequently the case. 

The spots have a wide range in size. Some are very 
minute points, barely visible in a good telescope, while on 
rare occasions one is large enough to be seen with the 
naked eye through a dark glass. They frequently ap- 
pear in groups, and a group may sometimes be made out 
with the naked eye as a minute patch when the individual 
spots cannot be seen. 

When the air is steady, and a good-sized spot is care- 
fully examined with a telescope, it will be seen to be com- 
posed of a dark central region or nucleus, surrounded 
by a shaded border. If all the conditions are favourable, 
this border will appear striated, like the edge of a 
thatched roof. The appearance is represented in the cut, 
which also shows the mottling of the photosphere. 

The spots are of the most varied and irregular forms, 
frequently broken up in many ways. The shaded border. 



96 



THE SUN, EARTH, AND MOON 



or the thatched lines which form it, frequently encroaches 
on the nucleus or maj^, in places, extend quite across it. 
A most remarkable law connected with the spots, which 
has been established by nearly three centuries of observa- 
tion, is that their frequency varies in a regular period of 
eleven years and about forty days. During a certain 
year no spots will be visible for about half of the time. 




Fig. 17. — Appearance of a Sun-spot with High Magnifying Poiver, show- 
ing also the Mottling of the Photosphere. 



This was the case in 1889 and again in 1900. The year 
following a slightly greater number will show themselves ; 
and they will increase year after year for about five 
years. Then the frequency will begin to diminish, year 
after year, until the cycle is completed, when it will again 
begin to increase. These mutations have been traced back 
to the time of Galileo, although it was not till about 1825 
that they were found by Schwabe to take place in a 
regular period. 



SPOTS ON THE SUN 



97 



Years of greatest and least frequency, past and future 
are as follows : 



Greatest 

1871 
1882 
1893 
1904 
1916 
1927 



Least 

1878 

1889 

1900 

1911 

1922 

1933 



Another noteworthy law connected with the sun's spots 
is that they are not found all over the sun; but only in 
certain regions of solar latitude. They are rather rare 
on the sun's equa- 
tor, but become 
more frequent as we 
go north or south 
of the equator till 
we get to fifteen de- 
grees of latitude, 
north or south. From 
this region to twen- 
ty degrees the fre- 
quency is greatest; 
then it falls off, so 
that beyond thirty 
degrees a spot is 
rarely seen. These 
regions are shown in 
the accompanying figure, where the shading is darker 




Fig. 18.— Frequency of Sun-spots in Differ- 
ent Latitudes on the Sun. 



98 THE SUN, EARTH, AND MOON 

the more frequent the spots. If we made a white globe 
to represent the sun, and made a black dot on it for every 
spot during a number of years, the dotting would make 
the globe look as represented in the figure. 

The Faculce 

Collections of numerous small spots brighter than the 
photosphere in general are frequentty seen on the sun. 
These are often seen in the neighbourhood of a spot, 
and occur most frequently in the regions of greater 
spot frequency, but are not entirely confined to those 
regions. They are, however, rare near the poles of 
the sun. 

That the spots and faculse proceed from some one 
general cause has been brought out by the spectro-helio- 
graph, an instrument devised by Professor George E. 
Hale for taking photographs of the sun by the light of 
a single ray of the spectrum, that emitted by calcium, for 
exarnple. The effect is the same as if we should look at 
the sun through a glass which would allow the rays of 
calcium vapour to pass, but would absorb all the others. 
We should then see the calcium light of the sun and no 
other. 

When the sun is photographed by calcium light with 
this instrument, the result is wonderful. The sun-spot 
regions are now seen to be brighter than the others, 
and faculae are found on every part of the sun. We thus 
learn that eruptions of gas, of which calcium is the best 
marked ingredient, are taking place all the time; but 
they are more numerous in the sun-spot zones than else- 



PROMINENCES AND CHROMOSPHERE 99 

where. The sun-spots are therefore the effect of opera- 
tions going on all the time, all over the sun, but giving 
rise to a spot only in the exceptional cases when they are 
very intense. 

It was formerly supposed that the spots were openings 
or depressions in the photosphere, showing a darker 
region within. Tliis view was based on the belief that, 
when a spot was near the edge of the sun's disk, the 
shaded border next the edge looked broader than the 
other. But this view is now abandoned. We cannot cer- 
tainl}" say that a spot is either above or below the photo- 
sphere. We shall hereafter see that the latter is not a 
mere surface as it seems to us, but a shell or covering 
many miles, perhaps a hundred or more, in thickness. 
The spots doubtless belong to this shell, being cooler por- 
tions of it, but lying neither above nor below it. 

The Prominences and Chromosphere 

The next remarkable feature of the sun to be described 
consists in the prominences. Our knowledge of these ob- 
jects has an interesting history — ^which will be mentioned 
in describing eclipses of the sun. The spectroscope 
shows us that large masses of incandescent vapour burst 
forth from every part of the sun. They are of such ex- 
tent that the earth, if immersed in them, would be as a 
grain of sand in the flame of a candle. They are thrown 
up with enormous velocity, sometimes hundreds of miles 
a second. Like the faculse, they are more numerous in 
the sun-spot zones, but are not confined to those zones. 
The glare around the sun caused by the reflection of light 



100 THE SUN, EARTH, AND MOON 

by the air renders them entirely invisible to vision, even 
with the telescope, except when, during total eclipses of 
the Sim, the glare is cut off* by the intervention of the 
moon. They may then be seen, even with the naked eye, 
rising up as if from the black disk of the moon. 

The prominences seem to be of two forms, the eruptive 
and the cloud-like. The first rise from the sun like im- 
mense sheets of flame ; the latter seem to be at rest above 
it, like clouds floating in the air. But there is no air 
around the sun for these objects to float in, and we can- 
not certainly say what supports them. Very likely, how- 
ever, it is a repulsive force of the sun's rays, which will 
be mentioned in a later chapter. 

Spectrum anal^^sis shows that these prominences are 
composed mostly of hydrogen gas, mixed with the va- 
pours of calcium and magnesium. It is to the hydrogen 
that they owe their red colour. Continued study of the 
prominences shows them to be connected with a thin layer 
of gases which surrounds and rests upon the photosphere. 
This layer is called the chromosphere, from its deep red 
colour, similar to that of the prominences. As in the case 
of the latter, most of its light seems to be that of hydro- 
gen; but it contains many other substances in seemingly 
varying proportions. 

The last appendage of the sun to be considered is the 
corona. This is seen only during total eclipses as a soft 
eff'ulgence surrounding the sun, and extending from it in 
long rays, sometimes exceeding the diameter of the sun 
in length. Its exact nature is still in doubt. It will be 
described in the chapter on eclipses. 



( 



HOW THE SUN IS MADE UP 101 

How the Sun is Made Up 

Let us now recapitulate what makes up the sun as we 
see and know it. 

We have first the vast interior of the globe which, of 
course, we can never see. 

What we see when we look at the sun is the shining 
surface of this globe, the photosphere. It is not a real 
surface, but more like!}" a gaseous layer several hundred 
miles deep which we cannot distinguish from a surface. 
This layer is variegated by spots, and in or over it rise 
the f aculse. 

On the top of the photosphere rests the layer of gases 
called the chromosphere, which can be observed at any 
time with a powerful spectroscope, but can be seen by 
direct vision only during total eclipses. 

Through or from the red chromosphere are thrown up 
the equally red flames called the prominences. 

Surrounding the whole is the corona. 

Such is the sun as we see it. What can we say about 
what it really is ? First, is it solid, liquid, or gaseous ? 

That it is not solid we have already shown by the law 
of rotation. It cannot be a liquid like molten metal, be- 
cause it sends off from its surface such a flood of heat as 
would cool off* and solidify molten metal in a very short 
time. For more than thirty years it has been understood 
that the interior of the sun must be a mass of gas, com- 
pressed to the density of a liquid by the enormous pres- 
sure of its superincumbent portions. But it was still sup- 
posed that the photosphere might be in the nature of a 



102 THE SUN, EARTH, AND MOON 

crust and the whole sun hke an immense bubble. This 
view, however, seems no longer tenable. It does not seem 
likely that there is any solid matter on the sun. 

Attempts have sometimes been made to learn the tem- 
perature of the photosphere. It probably exceeds any 
that we can produce on earth, even that of the electric 
furnace, else how could calcium, the metallic base of lime, 
one of the most refractor}^ of substances, exist there in 
a state of vapour? We all know that the air around us 
becomes cooler and rarer as we ascend above the surface 
of the earth, owing to the action of gravity and the con- 
sequent weight of the atmosphere, which gives rise to a 
constantly increasing pressure as we descend. Now, 
gravity at the sun is twenty-seven times as powerful as 
on the earth. Hence, going downward, temperature and 
pressure increase at a far more rapid rate on the sun 
than on the earth. Even in the photosphere the tempera- 
ture is such that "the elements melt with fervent heat." 
And, as we go below the surface, the heat must increase 
by hundreds of degrees for every mile that we descend.. 
The result is that in the interior the gases of the sun are 
subjected to two opposing forces w^hich grow more and 
more intense. These are the expansive force of the heat 
and the compressing force of the gases above, produced 
by the enormous force of gravity of the sun. 

The forces thus set in pla}^ merely in the outer portions 
of the sun's globe are simply inconceivable. Perhaps 
the explosion of the powder when a thirteen-inch cannon 
is fired is as striking an example of the force of ignited 
gases as we are familiar with. Now suppose every foot 



THE SUN'S HEAT 103 

of space in a whole county covered with such cannon, all 
pointed upward and all being discharged at once. The 
result would compare with what is going on inside the 
photosphere about as a boy's popgun compares with the 
cannon. 

The Source of the Sun'^s Heat 

Perhaps, from a practical point of view, the most com- 
prehensive and important problem of science is : How is 
the sun's heat kept up.^ Before the laws of heat were 
fully apprehended this question was not supposed to offer 
any difficulties. Even to this day it is supposed by those 
not acquainted with the subject, that the heat which we 
receive from the sun may arise in some way from the pas- 
sage of its rays through our atmosphere, and that, as a 
matter of fact, the sun may not radiate any actual heat 
at all — may not be an extremely hot body. But, modern 
science shows that heat cannot be produced except by 
the expenditure of some form of energy. The energy of 
the sun is necessarily limited in quantity and is continu- 
ally being lost through radiation. 

It is very easy to imagine the sun as being something 
like a white-hot cannon ball, which is cooling off by send- 
ing its heat in all directions, as such a ball does. We 
know by actual observation how much heat the sun sends 
to us. It may be expressed in the following way : 

Imagine a shallow basin with a flat bottom, and a 
depth of one centimetre, that Is, about four tenths of an 
inch. Let the basin be filled with water, the latter then 
being one centimetre deep. Expose such a basin to the 



104 THE SUN, EARTH, AND MOON 

rays of the vertical sun. The heat which the sun will 
radiate to them will be sufficient to warm the water about 
three and a half or four degrees Centigrade, or not very 
far from seven degrees Fahrenheit, in one minute. It 
follows that if we suppose a thin spherical shell of water, 
one centimetre thick, of the same radius as the earth's 
orbit, and having the sun in its centre, that shell of water 
will be heated with the rapidity just mentioned. The 
heat which it receives will be the total amount radiated 
by the sun. We can thus define how much heat the sun 
loses every minute, day and year. 

A very simple calculation will show that if the sun 
were of the nature of a white-hot ball it would cool off so 
rapidly that its heat could not last more than a few cen- 
turies. But it has in all probability lasted milhons of 
years. Whence, then, comes the supply? The answer 
of modern science to this question is that the heat radi- 
ated from the sun is supplied by the contraction of size 
as heat is lost. We all know that in many cases when mo- 
tion is destroyed heat is produced. When a cannon shot 
is fired at the armour plate of a ship of war, the mere 
stroke of the shot makes both plate and shot hot. The 
blacksmith can make iron hot by hammering it. 

These facts have been generalized into the statement 
that whenever a body falls and is stopped in its fall by 
friction, or by a stroke of any sort, heat is produced. 
From the law governing the case, we know that the water 
of Niagara, after it strikes the bottom of the falls, must 
be about one quarter of a degree warmer than it was 
during the fall. We also know that a hot body contracts 



THE SUN'S HEAT 105 

in volume when cooled. The contraction of a gaseous 
body, such as we believe the sun to be, is greater than 
that of a solid or liquid. The heat of the sun is radiated 
from streams of matter constantly rising from the in- 
terior, which radiate their heat when they reach the sur- 
face. Being cooled they fall back again, and the heat 
caused by this fall is what keeps the sun hot. 

It may seem almost impossible that heat sufficient to 
last for millions of years could be generated in this way ; 
but the known force of gravity at the surface of the sun 
enables us to make exact computations on the subject. 
It is thus found that in order to keep up the supply of 
heat it is only necessary that the diameter of the sun 
should contract about a mile in twenty-five years — or 
four miles in a century. This amount would not be per- 
ceptible until after thousands of years. Yet the process 
of contraction must come to an end some time. There- 
fore, if this view is correct, the life of the sun must have 
a limit. What its limit may be we cannot say with exact- 
ness, we only know that it is several millions of years, but 
not many millions. 

The same theory implies that the sun was larger in 
former times than it is now, and must have been larger 
and larger every year that we go back into its history. 
There was a time when it must have been as large as the 
whole solar system. In this case it could have been 
nothing but a nebula. We thus have the theory that the 
sun and solar sj'^stem have resulted from the contraction 
of a nebula — through millions of years. This view is 
familiarly known as the nebular hypothesis. 



106 THE SUN, EARTH, AND MOON 

The question whether the nebular hypothesis is to be 
accepted as a proved result of science is one on which 
opinions differ. There are many facts which support it 
■ — such as the interior heat of the earth and the revolu- 
tion and rotation of the planets all in the same direction. 
But cautious and conservative minds will want some fur- 
ther proof of the theory before they regard it as abso- 
lutely established. Even if we accept it, we still have 
open the question: How did the nebula itself originate, 
and how did it begin to contract ? This brings us to the 
boundary where science can propound a question but 
cannot answer it. 



< 



Ill 

The Earth 

The globe on which we live, being one of the planets, 
would be entitled to a place among the heavenly bodies 
even if it had no other claims on our attention. Insig- 
nificant though it is in size when compared with the great 
bodies of the universe, or even with the four giant planets 
of our system, it is the largest of the group to which it 
belongs. Of the rank which it might claim as the abode 
of man we need not speak. 

What is the earth .^^ We may describe it in the most 
comprehensive way as a globe of matter nearly eight 
thousand miles in diameter, bound together by the mu- 
tual gravitation of its parts. We all know that it is not 
exactly spherical, but bulges out very slightly at the 
equator. The problem of determining its exact shape 
and size is an extremely difficult one, and we cannot say 
that an entirely satisfactory result is yet reached. The 
difficulty is obvious enough. There is no way of measur- 
ing distances across the great oceans. The measurements 
are necessarily limited to such islands as are visible from 
the coasts of the continents or from each other. Of 
course, the measures cannot be extended to either pole. 
The size and shape must therefore be inferred from the 
measures across or along the continents. Owing to the 
importance of such work, the leading nations have from 



108 THE SUN, EARTH, AND MOON 

time to time entered into it. Quite recently our Coast 
and Geodetic Survey has completed the measurement of a 
line of triangles extending from the Atlantic to the 
Pacific Oceans. North and south measurements both on 
the Atlantic and Pacific coasts have been executed or are 
in progress. The English have from time to time made 
measures of the same sort in Africa, and the Russians 
and Germans on their respective territories. Nearly all 
these measures are now being combined in a work carried 
on by the International Geodetic Association, of which 
the geodetic authorities of the principal countries are 
members. 

The latest conclusions on the subject may be summed 
up thus. We remark in the first place that by the 
figure of the earth geodetists do not mean the figure 
of the continents, but of the ocean level as it would 
be if canals admitting the water of the oceans were 
dug through the continents. The earth thus defined is 
approximately an ellipsoid, of which the smaller diameter 
is that through the poles, and which has about the 
following dimensions : 

Polar diameter, 7,899.6 miles, or 12,713.0 kilometres. 

Equatorial " 7,926.6 miles, or 12,756.5 kilometres. 

It will be seen that the equatorial diameter is twenty- 
seven miles or forty-three kilometres greater than the 
polar. 

The Earth *s Interior 

What we know of the earth by direct observation is 
confined almost entirely to its surface. The greatest 
depth to which man has ever been able to penetrate com- 



THE EARTH'S INTERIOR 109 

pares with the size of the globe only as the skin of an 
apple does to the body of the fruit itself. 

I shall first invite the reader's attention to some facts 
about weight, pressure, and gravity in the earth. Let 
us consider a cubic foot of soil forming part of the outer 
surface of the earth. This upper cubic foot presses upon 
its bottom with its own weight, perhaps one hundred and 
fifty pounds. The cubic foot below it weighs an equal 
amount, and therefore presses on its bottom with a force 
equal to its own weight with the weight of the other foot 
added to it. This continual increase of pressure goes on 
as we descend. Every square foot in the earth's interior 
sustains a pressure equal to the weight of a column of 
the earth a foot square extending to the surface. Not 
many yards below the surface this pressure will be meas- 
ured in tons ; at the depth of a mile it may be thirty or 
forty tons ; at the depth of one hundred miles, thou- 
sands of tons ; continually increasing to the centre. Un- 
der this enormous pressure the matter composing the 
inner portion of the earth is compressed to the density of 
a metal. By a process which we will hereafter describe, 
the mean density of the earth is known to be five and one 
half times that of water, while the superficial density is 
only two or three times that of water. 

One of the most remarkable facts about the earth is 
that the temperature continually increases as we pene- 
trate below the surface in deep mines. The rate of in- 
crease is different in different latitudes and regions. The 
general average is one degree Fahrenheit in fifty or sixty 
feet. 



110 THE SUN, EARTH, AND MOON 

The first question to suggest itself is, how far toward 
the earth's centre does this increase of temperature ex- 
tend ? The most that we can say is that it cannot be 
merely superficial, because, in that case, the exterior por- 
tions would have cooled off long ago, so that we should 
have no considerable increase of heat as we went down. 
The fact that the heat has been kept up during the whole 
of the earth's existence shows that it must still be very 
intense toward the centre, and that the rate of increase 
near the surface must go on for many miles into the 
interior. 

At this rate the material of the earth would be red hot 
at a depth of ten or fifteen miles, while at one or two 
hundred miles the heat would be sufficient to melt all the 
substances which form the earth's crust. This fact sug- 
gested to geologists the idea that our globe is really a 
molten mass, like a mass of melted iron, covered by a cool 
crust a few miles thick, on which we dwell. The exist- 
ence of volcanoes and the occurrence of earthquakes 
gave additional weight to this view, as did also other 
geological evidence, showing changes in the earth's 
surface which appeared to be the result of a liquid 
interior. 

But in recent years the astronomer and physicist have 
collected evidence, which is as conclusive as such evidence 
can be, that the earth is solid from centre to surface, and 
even more rigid than a similar mass of steel. The sub- 
ject was first developed most fully by Lord Kelvin, who 
showed that, if the earth were a fluid, surrounded by a 
crust, the action of the moon would not cause tides in the 



EARTH'S GRAVITY AND DENSITY 111 

ocean, but would merely tend to stretch out the entire 
earth in the direction of the moon, leaving the relative 
positions of the crust and the water unchanged. 

Equally conclusive is the curious phenomenon which 
we shall describe presently of the variation of latitudes 
on the earth's surface. Not only a globe of which the 
interior is soft, but even a globe no more rigid than steel 
could not rotate as the earth does. 

How, then, are we to reconcile the enormous tempera- 
ture and the solidity? There seems to be only one solu- 
tion possible. The matter of the interior of the earth 
is kept solid by the enormous pressure. It is found ex- 
perimentally that when masses of matter like the rocks 
of the earth are raised to the melting point, and then 
subjected to heavy pressure, the effect of the pressure is 
to make them solid again. Thus, as we increase the tem- 
perature we have only to increase the pressure also to 
keep the material of the earth solid. And thus it is that, 
as we descend into the earth, the increase of pressure 
more than keeps pace with the rise of temperature, and 
thus keeps the whole mass solid. 

Gravity and Density of the Earth 

Another interesting question connected with the earth 
is that of its density, or specific gravity. We all know 
that a lump of lead is heavier than an equal lump of iron, 
and the latter heavier than an equal lump of wood. Is 
there any way of determining what a cubic foot of earth 
would weigh if taken out from a great depth of its vast 
interior .f^ If there is, then we can determine what the 



lis THE SUN, EARTH, AND MOON 

actual weight of the whole earth is. The solution de- 
pends on the gravitation of matter. 

Every child is familiar with gravitation from the time 
it begins to walk, but the profoundest philosopher knows 
nothing of its cause, and science has not discovered any- 
thing respecting it except a few general facts. The 
widest and most general of these facts, which may be said 
to include the whole subject, is Sir Isaac Newton's theory 
of gravitation. According to this theory, the mysterious 
force by which all bodies on the surface of the earth tend 
to fall toward its centre does not reside merely in the 
centre of the earth, but is due to an attraction exerted 
by every particle of matter composing our globe. 
Whether this was the case was at first an open question. 
Even so great a philosopher and physicist as Huyghenc 
believed that the power resided in the earth's centre, and 
not in every particle, as Newton supposed. But the lat- 
ter extended his theory yet farther by showing that every 
particle of matter in the universe, so far as we have yet 
ascertained, attracts every other particle with a force 
that diminishes as the square of the distance increases. 
This means that at twice the distance the attraction will 
be divided by four ; at three times by nine ; at four times 
by sixteen, and so on. 

Granting this, it follows that all objects around us 
have their own gravitating power, and the question 
arises : Can we show this power by experiment, and meas- 
ure its amount .P The mathematical theory shows that 
globes should attrac: small bodies at their surfaces with 
a force proportioned to their diameter. A globe two 



ATTRACTION OF THE EARTH 113 

feet in diameter, of the same specific gravity as the earth, 
should attract with a force one twenty-milHonth of the 
earth's gravity. 

In recent times several physicists have succeeded in 
measuring the attraction of globes of lead having a 
diameter of a foot, more or less. This measurement is 
the most delicate and difficult that has ever been made, 
and the accuracy which seems to have been reached would 
have been incredible a few years ago. The apparatus 
used is, in its principle, of the simplest kind. A very 
light horizontal rod is suspended at its centre by a thread 
of the finest and most flexible material that can be ob- 
tained. This rod is balanced by having a small ball at- 
tached to each end. What is measured is the attraction 
of the globes of lead upon these two balls. The former 
are placed in such a position as to unite their attraction 
in giving the rod a slight twisting motion in the horizon- 
tal plane. To appreciate the difficulties of the case, we 
must call to mind that the attraction may not amount to 
the ten-millionth part of the weight of the little balls. 
It would be difficult to find any object so light that its 
weight would not exceed this force. To compare the 
weight of a fly with it would be like comparing the 
weight of an ox with that of a dose of medicine. Not 
only the weight of a mosquito but even of its finest limb 
might exceed the quantity to be measured. If a mosquito 
were placed under a microscope an expert operator could 
cut off from one antenna a piece small enough to express 
the force measured. 

Yet the determination of this force has been made with 



114 THE SUN, EARTH, AND MOON 

such precision that the results of the two latest investiga- 
tors do not differ by a thousandth part. These were 
Professor Boys, F.R.S., of Oxford, England, and 
Dr. Karl Braun, S.J., of Marienschein, in Bohemia. 
They worked independently at the problem, meeting 
and overcoming innumerable difficulties one after another, 
getting greater and greater delicacy and precision in 
their apparatus, and finally published their results al- 
most at the same time, the one in England, the other in 
Austria. The outcome of their experiments is that the 
mean density of the earth is slightly more than five and 
a half times that of water. This is a little less than the 
density of iron, but much more than that of any or- 
dinary stone. As the mean density of the materials 
which compose the earth's crust is scarcely more than 
one half of this amount, it follows that near the centre 
the matter composing the earth must be compressed to 
a density not only far exceeding that of iron, but prob- 
ably that of lead. 

The attraction of mountains has been known for more 
than a hundred years. It was first demonstrated by 
Maskelyne about 1775 in the case of Blount Schehallion, 
in Scotland. In all mountain regions where very accu- 
rate surveys are made the attraction of mountains upon 
the plumb line is very evident. 

Variations of Latitude 

We know that the earth rotates on an axis passing 
through the centre and intersecting the earth's surface at 
either pole. If we imagine ourselves standing exactly 



VARIATIONS OF LATITUDE 115 

on a pole of the earth, with a flagstaff fastened in the 
ground, we should be carried round the flagstaff by the 
earth's rotation once in twenty-four hours. We should 
become aware of the motion by seeing the sun and stars 
apparently moving in the opposite direction in horizontal 
circles by virtue of the diurnal motion. Now, the great 
discovery of the variation of latitude is this : The point 
in which the axis of rotation intersects the surface is not 
fixed, but moves around in a somewhat variable and ir- 
regular curve, contained within a circle nearly sixty feet 
in diameter. That is to say, if standing at the north 
pole we should observe its position day by day, we should 
find it moving one, two, or three inches every day, de- 
scribing in the course of time a curve around one central 
point, from which it would sometimes be farther away 
and sometimes nearer. It would make a complete revolu- 
tion in this irregular way in about fourteen months. 

Since we have never been at the pole, the question 
may arise: How is this known? The answer is that by 
astronomical observations we can, on any night, deter- 
mine the exact angle between the plumb line at the place 
where we stand and the axis on which the earth is rota- 
ting on that particular day. Four or five stations for 
making these observations were established around the 
earth in 1900 by the International Geodetic Association. 
One of these stations is near Gaithersburg, Md., another 
is on the Pacific coast, a third is in Japan, and a fourth 
in Italy. Before these were established observations 
having the same object were made in various parts of 
Europe and America. The two most important stations 



116 THE SUN, EARTH, AND MOON 

in the latter region were those of Professor Rees of Co- 
lumbia University, New York, and of Professor Doolittle, 
first at Lehigh, and later at the Flower Observatory, 
near Philadelphia. 

The variation which we have described was originally 
demonstrated by S. C. Chandler, of Cambridge, in 1890 
by means of a great mass of astronomical observations 
not made for this special purpose. Since then investi- 
gation has been going on with the view of determining 
the exact curve described. What has been shown thus 
far is that the variation is much wider some years than 
others, being quite considerable in 1891, and very small 
in 1894. It appears that in the course of seven years 
there will be one in which the pole describes the greater 
part of a comparatively wide circle, while three or four 
years later it will for several months scarcely move from 
its central position. 

If the earth were composed of a fluid, or even of a 
substance which would bend no more than the hardest 
steel, such a motion of the axis as this would be impossi- 
ble. Our globe must therefore, in the general average, 
be more rigid than steel. 

The Atmosphere 

The atmosphere is astronomically, as well as physic- 
ally, a most important appendage of the earth. Neces- 
sary though it is to our life it constitutes one of the 
greatest obstructions with which the astronomer has to 
deal. It absorbs more or less of all the light that passes 
through it, and thus slightly changes the colour of the 



THE ATMOSPHERE 117 

heavenly objects as we see them, and renders them some- 
what dimmer, even in the clearest sky. It also refracts 
the light passing through it, causing it to describe a 
slightly curved line, concave toward the earth, instead of 
passing straight to the astronomer's eye. The result of 
this is that the stars appear slightly higher above the 
horizon than they actually are. The light coming direc i ly 
down from a star in the zenith suffers no refraction. The 
latter increases as the star is farther from the zenith, 
but even forty-five degrees away it is only one minute 
of arc, about the smallest amount that the unaided eye 
can plainly perceive; yet this is a very important quan- 
tity to the astronomer. The nearer the object is to the 
horizon the greater the rate at which the refraction in- 
creases ; twenty-eight degrees above the horizon it is 
about twice as great as at forty-five degrees ; at the hori- 
zon it is more than one half a degree, that is more than 
the whole diameter of the sun or moon. The result is that 
when we see the sun just about to touch the horizon at 
sunset or sunrise its whole body is in reality below the 
horizon. We see it only in consequence of the refraction 
of its light. Another result of the rapid increase near 
the horizon is that, in this position, the sun looks decid- 
edly flattened to the eye, its vertical diameter being 
shorter than the horizontal one. Anyone may notice 
this who has an opportunity to look at the sun as it is 
setting in the ocean. It arises from the fact that the 
lower edge of the sun is refracted more than the upper 
edge. 

When the sun sets in the ocean in the clear air of the 



118 THE SUN, EARTH, AND MOON 

tropics a beautiful effect may be noticed, which can 
rarely or never be seen in the thicker air of our latitudes. 
It arises from the unequal refraction of the rays of light 
by the atmosphere. Like a prism of glass the atmos- 
phere refracts the red rays the least and the successive 
spectral colours, yellow, green, blue, and violet, more 
and more. The result is that, as the edge of the sun is 
disappearing in the ocean, these successive rays are lost 
sight of in the same order. Two or three seconds before 
the sun has disappeared^ the little spark of its limb 
which still remains visible is seen to change colour and 
rapidly grow paler. This tint changes to green and 
blue, and finally the last glimpse which we see is that of 
a disappearing flash of blue or violet light. 



IV 
The Moon 

About one hundred years ago there was an unpopular 
professor in the Government Polytechnique School of 
Paris, still the great school of mathematics for the 
French public service, who loved to get his students into 
difficulties. One morning he addressed one of them the 
question : 

"Monsieur, have you ever seen the moon?" 

"No, sir," replied the student, suspecting a trap. 

The professor was nonplussed. "Gentlemen," said he, 

"see Mr. , who profes^s never to have seen the 

moon !" 

The class all smiled. 

"I admit that I have heard it spoken of," said the 
student, "but I have never seen it." 

I take it for granted that the reader has been more 
observant than the French student professed to be, and 
that he has not only seen the moon, but knows the phases 
through which it goes and is familiar with the fact that 
it describes a monthly course around the earth. I also 
suppose that he knows the moon to be a globe, although, 
to the naked eye, it seems like a flat disk. The globular 
form is, however, very evident when we look at it with a 
small telescope. 

Various methods and systems of measurement all agree 



120 THE SUN, EARTH, AND MOON 

in placing the moon at an average distance of a little 
less than two hundred and forty thousand miles. This 
distance is obtained by direct measure of the paral- 
lax, as will be explained hereafter, and also by calcula- 
ting how far off the moon must be in order that, being 
projected into space, it may describe an orbit around the 
earth in the time that it actually does perform its 
round. The orbit is elliptic, so that the actual distance 
varies. Sometimes it is ten or fifteen thousand miles less, 
at other times as much more, than the average. 

The diameter of the moon's globe is a little more than 
one fourth that of the earth; more exactly, it is two 
thousand one hundred and sixty miles. The most careful 
measures show no deviation from the globular form 
except that the surface is very irregular. 

Revolution and Phases of the Moon 

The moon accompanies the earth in its revolution 
round the sun. To some the combination of the two 
motions seems a little complex ; but it need not offer any 
real difficulty. Imagine a chair standing in the centre 
of a railway car in rapid motion, while a person is walk- 
ing around it at a distance of three feet. He can go 
round and round without varying his distance from the 
chair and without any difficulty arising from the motion 
of the car. Thus the earth moves forward in its orbit, 
and the moon continually revolves around it without 
greatly varying its distance from us. 

The actual time of the moon's revolution around the 
earth is twenty-seven days eight hours; but the time 



MOON'S REVOLUTION AND PHASES 121 

from one new moon to another is twenty-nine days thir- 
teen hours. The difference arises from the earth's mo- 
tion around the sun ; or, which amounts to the same thing, 
the apparent motion of the sun along the echptic. To 



m 



I 



1 \ 

i \ 

! \ 

! \ 
I \ I 

I \ / 

/ 



\ / 



»MOON V. 



'4' 



MOON 






\ 



^ARTH /^-^\ eAB-f^^ 



^- ^•■ 

Fig. 19. — Revolution of the Moon Round the Earth. 

show this, let AC be a small arc of the earth's orbit 
around the sun. Suppose that at a certain time the earth 
is at the point E, and the moon at the point M, between 
the earth and the sun. At the end of twenty-seven days 
eight hours the earth will have moved from E to F. 



122 THE SUN, EARTH, AND MOON 

While the earth is making this motion the moon will have 
moved around the orbit in the direction of the arrows, 
so as to have reached the point N. At the moment when 
the lines E]\I and FN are parallel to each other, the moon 
will have completed her actual revolution, and will seem 
to be in the same place among the stars as before. But 
the sun is now in the direction FS. The moon therefore 
has to continue its motion before it catches up to the sun. 
This requires a little more than two days, and makes 
the whole time between two new moons twenty-nine and 
a half days. 

The varying phases of the moon depend upon its 
position with respect to the sun. Being an opaque globe, 
without light of its own, we see it only as the light of 
the sun illuminates it. When it is between us and the 
sun its dark hemisphere is turned toward us, and it is 
entirely invisible. The time of this position in thie 
almanacs is called "new moon," but we cannot commonly 
see the moon for nearly two days after this time, because 
it is lost in the bright twilight of evening. On the second 
and third day, however, we see a small portion of the 
illuminated globe, having the familiar form of a thin 
crescent. This crescent we commonly call the new moon, 
although the time given in the almanac is several days 
earlier. 

In this position, and for several days longer, we may, 
if the sky is clear, see the entire face of the moon, the 
dark parts shining with a faint gray light. This light 
is that which is reflected from the earth to the moon. 
An inhabitant of the moon, if there were such, would 



SURFACE OF THE MOON 123 

then see the earth in the skj hke a full moon, looking 
much larger than the moon looks to us. As the moon 
advances in its orbit day after day, this light diminishes, 
and about the time of first quarter disappears from our 
sight owing to the brightness of the illuminated portion 
of the moon. 

Seven or eight days after the almanac time of new 
moon, the moon reaches its first quarter. We then see 
half of the illuminated disk. During the week following, 
the moon has the form called gibbous. At the end of the 
second week the moon is opposite the sun, and we see its 
entire hemisphere like a round disk. This we call full 
moon. During the remainder of its course the phases 
recur in reverse order, as we all know. 

We might regard all these recurrences as too well 
known to need description, yet, in the Ancient Mariner, 
a star is described as seen between the two horns of the 
moon as though there were no dark body there to inter- 
cept our view of the star. Probably more than one poet 
has described the new moon as seen in the eastern sky, 
or the evening full moon as seen in the west. 

The Surface of the Moon 

We can see with the naked eye that the moon's surface 
is variegated by bright and dark regions. The latter 
are sometimes conceived to have a vague resemblance to 
the human face, the nose and eyes being especially prom- 
inent. Hence the "man in the moon." Through even 
the smallest telescopes we see that the surface has an im- 
mense variety of detail ; and the more powerful the tele- 



1^4 THE SUN, EARTH, AND MOON 



ft- y >.;«-,€■■% .. -'' 




-^r 



Fig. 20. — Mountainous Surface of the Moon. 



SURFACE OF THE MOON 125 

scope the more details we see. The first thing to strike 
us on a telescopic examination will be the elevations, or 
mountains as they are commonly called. These are best 
seen about the time of the first quarter, because they then 
cast shadows. At full moon they cannot be so well made 
out, because we are looking straight down and see every- 
thing illuminated. Although these elevations and de- 
pressions are called mountains they are different in 
form from the ordinary mountains of the earth. 
There is, however, an almost exact resemblance be- 
tween them and the craters of our great volcanoes. 
A very common form is that of a circular fort, one 
or more miles in diameter, with walls which may be 
thousands of feet high. The inside of this fort may 
be saucer shaped, a large portion of the surface being 
flat. At first quarter we can see the shadow of the walls 
cast upon the interior flat surface. In the centre a ^ttle 
cone is frequently seen. The interior surface is by no 
m.eans perfectly flat and smooth. The higher power the 
more details we shall see. Just what these consist of it is 
impossible to say ; they may be solid rock or they may be 
piles of loose stone. As we can see no object on the moon, 
even with the most powerful telescope, unless it is more 
than a hundred feet in diameter, we cannot say what the 
exact nature of the surface is in its minutest portions. 

The early observers with the telescope supposed that 
the dark portions were seas and the brighter portions 
continents. This notion was founded on the fact that the 
darker portions looked smoother than the others. Names 
were therefore given to these supposed oceans, such 



126 THE SUN, EARTH, AND MOON 

as Mare Procellarum, the Sea of Storms ; Mare Serenita- 
tis, the Sea of Cahns, etc. These names, fanciful though 
they be, are still retained to designate the large dark 
regions on the moon. A very sHght improvement in the 
telescope, however, showed that the idea of these dark 
regions being oceans was an illusion. They are all cov- 
ered with inequalities, proving that they must be com- 
posed of solid matter. The difference of aspect arises 
from the lighter or darker shade of the materials which 
compose the lunar surface. These are distributed over 
the surface of the moon in a very curious way. One of 
the most remarkable features are the long bright lines 
which radiate from certain points on the moon. A very 
low telescopic power will show the most remarkable of 
these; a good eye might even perceive it without a tele- 
scope. On the southern part of the moon's hemisphere, 
as we see it, is a large spot or region known as Tycho, 
and from this radiate a number of these bright streaks. 
The appearance is as if the moon had been cracked and 
the cracks filled up with melted white matter. 

Whether we accept this view or not, it is impossible to 
examine the surface of the moon without the conviction 
that in some former age it was the seat of great volcanic 
activity. In the centre of all the great circular moun- 
tains we have described are craters which, it would seem, 
must have been those of volcanoes. Indeed, a hundred 
years ago it was supposed by Sir William Herschel that 
there was an active volcano on the moon, but it is now 
known that this appearance is due to the light of the 
earth reflected from a very bright spot on the moon's 



AIR OR WATER ON THE MOON? 127 

surface. It can be easily seen about the time of the new 
moon with a telescope of moderate size. 

Is there Air or Water on the Moon? 

One of the most important questions connected with 
the moon is whether there is any air or water on its sur- 
face. To these the answer of science up to the present 
time is in the negative. Of course this does not mean 
that there can absolutely not be a drop of moisture nor 
the smallest trace of an atmosphere on our satellite; all 
we can say is that if any atmosphere surrounds the moon 
it is so rare that we have never been able to get any evi- 
dence of its existence. If the latter had such an append- 
age of even one hundredth of the density of the earth's 
atmosphere, its existence would be made known to us by 
refraction of the Hght from a star seen alongside the 
moon. But not the slightest trace of any such refrac- 
tion can be discovered. If there is any such liquid as 
water, it must be concealed in invisible crevices, or diffused 
fused through the interior. Were there any large sheets 
of water in the equatorial regions they would reflect the 
light of the sun day by day, and would thus become 
clearly visible. The water would also evaporate and form 
more or less of an atmosphere of watery vapour. 

All this seems to settle another important question; 
namely, that of the habitability of the moon. Life, in 
the form in which it exists on our earth, requires water 
at least for its support, and in all its higher forms air 
also. We can hardly conceive of a living thing made of 
mere sand or other dry matter such as forms the lunar 



128 THE SUN, EARTH, AND MOON 

surface. If we supposed animals to walk about on the 
moon, it is difficult to imagine what they could eat. Our 
general conclusion must be that there is no life on the 
moon subject to the laws which govern life on the surface 
of this earth. 

The total absence of air and water results in a state of 
things on the moon such as we never experience on the 
earth. So far as can be ascertained by the most careful 
examination, not the slightest change ever takes place 
on its surface. A stone lying on the surface of the earth 
is continually attacked by the weather and in the course 
of years is gradually disintegrated or washed away by 
the wind and water. But there is no weather on the moon, 
and a stone lying on its surface might rest there for un- 
known ages undisturbed by any cause whatever. The 
lunar surface is heated up when the sun shines on it and 
it cools oif when the sun has set. Except for these 
changes of temperature there is absolutely nothing going 
on over the whole surface of the moon, so far as we can 
see. A world which has no weather and on which nothing 
ever happens — such is the moon. 

Rotation of the Moon 

The rotation of the moon on its axis is a subject on 
which some are frequently so perplexed that we shall 
explain it. Anyone who has carefully examined this 
body knows that it always presents the same face to us. 
This shows that it rotates on its axis in the same time 
that it revolves around the earth. An idea frequently 
entertained is that this shows that it does not rotate at all. 



HOW THE MOON PRODUCES TIDES 129 

and many chapters have been written on this subject. 
The whole difficulty arises from the different ideas which 
people have of motion. In physics we say that a body 
does not rotate when, if a rod were passed through it, 
that rod always maintained the same direction when the 
body moved about. 
Now let us sup- 
pose such a rod 
passed through the 
moon ; then, if the 
latter did not ro- 
tate on its axis the 
rod would main- 
tain its same direc- 
tion while the 
moon, revolving 
around the earth, 
would appear at 
different points in 

its orbit as we see it in Figure 21. A very little study 
of this figure will show that as the moon went around 
we should successively see every part of its surface 
in succession if it did not rotate on its axis. 




^^>-^ 



Fig. 21. 



-Showing hoiu the Moon would Move if 
it did ttot Rotate on its Axis. 



How the Moon Produces the Tides 

All of us who live on the seashore know that there is 
a rise and fall of the ocean which in the general average 
occurs about three quarters of an hour later every day, 
and which keeps pace with the apparent diurnal motion 
of the moon. That is to say, if it is high tide to-day when 



130 THE SUN, EARTH, AND MOON 

the moon is in a certain position in the heavens, it will be 
high tide when the moon is in or near that position day 
after day, month after month, and year after year. We 
have all heard that the moon produces these tides by its 
attraction on the ocean. We readily understand that 
when the moon is above any region its attraction tends 
to raise the waters in that region; but the circumstance 
that most perplexes those who are not expert in the sub- 
ject is that there are two tides a day, high tide occurring 
not only under the moon, but on the side of the earth 
opposite the moon. The explanation of this is that the 
moon really attracts the earth itself as well as it does 
the water. It continually draws the entire earth and 
everything upon it toward itself. As it goes round the 
earth in its monthly course, it thus keeps up a continual 
motion of the latter. If it attracted every part of the 
earth equally, the ocean included, there would then be 
no tides, and everything would go on on the earth's sur- 
face as if there were no attraction at all. But as the 
attraction is as the inverse square of the distance, the 
moon attracts the regions of the earth and oceans which 
are nearest to it more than the average, and those that 
are farthest from it less than the average. 

To show the effect of these changes let A, C, and H be 
the three points on the earth attracted by the moon. 
Since the moon attracts C more than A, it tends to pull 
C away from A and increase the distance between A and 
C. At the same time pulling H more than C it tends to 
increase the distance between H and C. If the whole earth 
was a fluid, the attraction of the moon would be simply to 



HOW THE MOON PRODUCES TIDES 131 

draw this fluid out into the form of an ellipsoid, of which 
the long diameter would be turned toward the moon. But 
the earth itself, being solid, cannot be drawn out into tliis 
shape, while the ocean, being fluid, is thus drawn out. 
The result is that we have high tides at the two ends of 
the ellipse into which the ocean is drawn, and low tides 
in the mid-region. 

The complete explanation of the subject requires a 
statement of the laws of motion which cannot be made 



MOON 
LINE OF PULL 




•O 



Fig. 22. — How the Moon's Pull on the Earth and Ocean Produces Tiuo 
Tides in a Day, 

here. I will, however, remark that if the attraction of 
the moon on the earth were always in the same direction, 
the two bodies would be drawn together in a few days. 
But owing to the revolution of the moon round the earth 
the direction of the pull is always changing, so that the 
earth is, in the course of a month, only drawn about 
three thousand miles from its mean position by the 
moon's pull. 

It might be supposed that if the moon produces the 
tides in this way we should always have high tide when 
the moon is on the meridian and low tide when the moon 
is in the horizon. But such is not the case, for two rea- 
sons. In the first place it takes time for the moon to draw 



132 THE SUN, EARTH, AND MOON 

the waters out into the form of an elHpsoid, and when 
it once gives them the motion necessary to keep this form, 
that motion keeps up after the moon has passed the 
meridian, just as a stone continues to rise after it has left 
the hand or a wave goes forward by the momentum of 
the w^ater. The other cause is found in the interruption 
of the motion by the great continents. The tidal w^ave, 
as it is called, meeting a continent, spreads out in one 
direction or the other, according to the lay of the land, 
and may be a long time in passing from one point to 
another. Thus arise all sorts of irregularities in the 
tides when we compare those in different places. 

The sun produces a tide as well as the moon, but a 
smaller one. At the times of new and full moon the 
two bodies unite their forces and cause the highest and 
lowest tides. These are familiar to all dwellers on the 
seacoast and are called spring tides. About the time 
of the first and last quarters the attraction of the sun 
opposes that of the moon and the tides do not rise so 
high or fall so low, and these are called neap tides. 



V 

Eclipses of the Moon 

The reader is doubtless aware that an eclipse of the 
moon is caused by that body entering the shadow of the 
earth, and that an eclipse of the sun is caused by the 
moon passing between us and the sun. Taking this 
knowledge for granted, we shall explain the more inter- 
esting features of these phenomena and the laws of their 
recurrence. 

The first question to be considered is: Why is there 
not an eclipse of the moon at every full moon, since the 
earth's shadow must always be in its place opposite the 



'~~--4---.,< ::-''-' 



Fig. 23. — The Moon in the Shadow of the Earth. 

sun.f^ The answer is that the moon commonly passes 
either above or below the shadow of the earth, and so fails 
to be eclipsed. This, again, arises from the fact that 
the orbit of the moon has a small inclination, about five 
degrees, to the plane of the ecliptic, in which the earth 
moves, and in which the centre of the shadow always lies. 
Returning to our former thought of the ecliptic being 




134 THE SUN, EARTH, AND MOON 

marked out on the celestial sphere, let us suppose that 
we also mark out the orbit of the moon during the course 
of its monthly period. We should then find the orbit of 
the moon crossing that of the sun in two opposite points, 
at the very small angle of five degrees. These points of 
crossing are called nodes. At one node the moon passes 
from below, or. south of the ecliptic, to the north of it. 
This is called the ascending node. At the other the 
moon passes from north to south of the ecliptic. This is 
called the descending node. The terms ascending and 
descending are applied to the node, because to us in the 
northern hemisphere, the north side of the ecliptic and 
equator seem to be above the south side. 

At the points halfway between the nodes the centre 
of the moon is above the ecliptic by about one twelfth its 
distance from us, that is, by about twenty thousand miles. 
The sun being larger than the earth, the shadow of the 
latter gradually grows smaller away from the earth. At 
the distance of the moon its diameter is about three 
fourths that of the earth, that is about six thousand 
miles. Its centre being in the plane of the ecliptic, it 
extends only about three thousand miles above and below 
that plane. Hence it is that the moon will pass through 
it only when near the nodes. 

Eclipse Seasons 

The line joining the sun and moon of course turns 
round as the earth moves around the sun. It therefore 
crosses the moon's nodes twice in the course of a year. 
That is to say if we supose the nodes to be marked in the 



ECLIPSE SEASONS 135 

sky, the ascending node at one point, and the descending 
node at the opposite point, then the sun will appear to 
us to pass each of these points in the course of a year. 
While the sun is passing one node the shadow of the 
earth will seem to be passing the other. It is only near 
these two times of the year that an eclipse of the sun or 
moon can occur. We may therefore call them eclipse 
seasons. They commonly last about a month; that is 
to say it is generally about a month from the time when 
the sun gets near enough to a node to allow of an 
eclipse until the time when it is too far past for an 
eclipse to occur. In 1901 the seasons were May and 
November. 

If the moon's node stayed in the same place in the sky^ 
eclipses would occur only some time during these two 
months. But, owing to the attraction of the sun on the 
earth and moon, the position of the nodes is continually 
changing in a direction opposite that of the motion of 
the two bodies. Each node makes a complete revolution 
around the celestial sphere in eighteen years and seven 
months. Hence in this same period the eclipse seasons 
will course all through the year. On an average they 
occur about nineteen days earlier every year than they 
did the year before. Thus it happens that in 1903 one 
season occurs in March and April and the other season in 
September and October. The change will keep going on 
until, in the year 1910, the season which in 1901 was in 
May will have gotten back to November, while the No- 
vember one will have gotten back to May, each having 
passed through all the intermediate months, and the two 



13o THE SUN, EARTH, AND MOON 



having changed places. By 1919 each will have made 
an entire revolution through the year. 

Let us imagine ourselves to be looking at the sun and 
earth from the moon when the latter is about to enter the 
earth's shadow. The earth, looking much larger than 
the sun, will be seen to approach it, and at length will 
begin to impinge on its disk and cut off a part of its 
light. The region within which this w411 occur is called 
the penumhra, and it is shown outside the shadow in the 
figure. So long as the moon is only in this region, an 





Fig. 24. — Passage of the Moon throtigh the Earths Shadow. 

ordinary observer would not notice any diminution in its 
light, although such a diminution could be detected by 
exact photometric measurements. The moon is not said 
to be eclipsed until it begins to enter into the actual 
shadow, where the whole direct light of the sun is cut off. 

How an Eclipse of the Moon Looks 

If we watch the moon when an eclipse is about to be- 
gin, we shall see a small portion of her eastern edge grad- 
ually grow dim and finally disappear. As the moon 
advances in her orbit, more and more of her face thus 
disappears from view by entering into the shadow. If, 
however, we look very carefully, we shall see that the part 



HOW AN ECLIPSE OF MOON LOOKS '^137 

immersed in the shadow has not entirely disappeared, but 
shines with a very faint Hght. If the whole body of the 
moon enters into the shadow, the eclipse is said to be 
total ; if only a portion of her body dips into the shadow, 
it is called partial. If the eclipse is total, the light which 
illuminates the eclipsed moon will be very plainly seen, 
because it is not drowned out by the dazzling light of the 
uneclipsed portion. This light is of a dingy red colour, 
and arises from the refraction of the earth's atmosphere, 
which was described in a former chapter. In consequence 
of this, those rays of the sun which just graze the earth, 
or pass within a short distance of its surface, are bent out 
of their course and thrown into the shadoAV by refraction. 
Thus they fill the shadow and fall on the moon. The red 
colour is due to the same cause that makes the sun appear 
red at sunset, namely, the absorption of the green and 
blue rays by the atmosphere, which lets the red rays pass. 

Two or three eclipses of the moon occur every year, of 
which one, at least, is nearly always total. But, of 
course, the eclipse will be visible only in that hemisphere 
of the earth on which the moon is shining at the time. 

When the moon is eclipsed an observer on that body 
would see an eclipse of the sun by the earth. The cause 
of the phenomenon we have described would then be plain 
enough to him. The apparent size of the earth would 
be much larger than that of the moon as we see it. Its 
diameter would be between three and four times that of 
the sun. At first this immense body would be invisible 
when it approached the sun. What the observer would 
see would be the cutting off of the light of the sun by the 



138 THE SUN, EARTH, AND MOON 

advancing but inTisible earth. When the latter had 
nearly covered the sun, its whole outline would be shown 
to liim by a red light surrounding it, caused by the re- 
fraction of the earth's atmosphere. Finally, when the 
last trace of true sunlight had disappeared, nothing 
would be visible but this ring of bright red light having 
inside of it the black but otherwise invisible body of the 
earth. 

The circumstances of an eclipse of the moon are quite 
different from those of a solar eclipse, to be described in 
the next chapter. It can aways be seen at the same in- 
stant over the whole hemisphere of the earth on which 
the moon is shining at the time. A curious phenomenon 
occurs when the moon rises totally eclipsed. Then we 
may see it on one horizon, say the eastern one, while the 
sun is still visible on the western horizon. The explana- 
tion of this seeming paradox is that both bodies are reall}'^ 
below the horizon, but are so elevated by refraction that 
we can see them at the same time. 



VI 

Eclipses of the Sun 

If the moon moved exactly in the plane of the ecliptic 
she would pass over the face of the sun at every new 
moon. But, owing to the inclination of her orbit, as de- 
scribed in the preceding chapter, she will actually do so 
only when the direction of the sun happens to be near one 
of the moon's nodes. When this is the case we may see 
an eclipse of the sun if we are only on the right part of 
the earth. 

Supposing the moon to pass over the sun, the first 
question is whether it can wholly hide the sun from our 
eyes. This depends not on the actual size of the two 





Fig. Ih.— TJie Shadow of the Moon TJirown on the Earth during a Total 
Eclijjse of the Sun. 

bodies but on their apparent size. We know that the sun 
has about four hundred times the diameter of the moon. 
But it is also four hundred times as far from us as the 
moon. The curious result of this Is that the two bodies 
appear of nearly the same size to our eyes. Sometimes 



140 THE SUN, EARTH, AND MOON 

the moon appears a little the larger, and sometimes the 
sun. In the former case the moon may entirely hide the 
sun ; in the latter case she cannot do so. 

One important difference between an eclipse of the 
moon and of the sun is that the former is always the same 
wherever it is Aasible, while an eclipse of the sun depends 
upon the position of the observer. The most interesting 
eclipses are those in which the centre of the moon passes 
exactly over that of the sun. These are called central 



^^^^^^^M 






^^B 


^^p 


^^^^^^^^^^^^^^^^^ 




^^^^^^iB^^ 


^^^^^^ 


^ ^^^1^ 


^^^S 




^ft 


ft 


i 


^SB^^^^^^^ 






^^^p 


^^^H 


^^^^^^^^^^^^^^^^ 






^^^^^ 


^^^^^^ 



Fig. 26o — T/ie Moon Passing Centrally over the Sun during an Annular 

Eclipse. 

eclipses. To see one, the observer must station himself 
at a point through which the line joining the centres 
shall pass. Then if the apparent size of the moon ex- 
ceeds that of the sun, the former will completely hide the 
sun from view. The eclipse is then said to be total. 

If the sun appears the larger, a ring of its light will 
surround the dark body of the moon at the moment of 
central eclipse. The latter is then called annular (Latin 
annulus, a ring). 

The line of centres of the two bodies sweeps along the 
surface of the earth, and its course may be shown by a 
line marked on a map. Such maps, showing the regions 



BEAUTY OF A TOTAL ECLIPSE 141 

and lines of eclipses are published in the astronomical 

ephemerides. An eclipse may be total or annular in a 

region a few miles north or south of this central line, but 

never for so far as one hundred miles. Outside this 

hmit an observer will see only a partial eclipse, that is, 

one in which the moon partly covers the sun. In yet 

more distant regions of the earth there will be no eclipse 

at all. 

Beauty of a Total Eclipse 

A total eclipse is one of the most impressive sights that 
nature offers to the eye of man. To see it to the best 
advantage one should be in an elevated position com- 
manding the widest possible view of the surrounding 
country, especially in the direction from which the 
shadow of the moon is to come. The first indication of 
anything unusual is to be seen, not on the earth or in the 
air, but on the disk of the sun. At the predicted moment 
a little notch will be seen to form somewhere on the west- 
ern edge of the sun's outline. It increases minute by 
minute, gradually eating away, as it were, the visible 
sun. No wonder that imperfectly civilised people, when 
they saw the great luminary thus diminishing in size, 
fancied that a dragon was devouring its substance. 

For some time, perhaps an hour, nothing will be 
noticed but the continued progress of the advancing 
moon. It will be interesting if, during this time, the ob- 
server is in the neighbourhood of a tree that will permit 
the sun's rays to reach the ground through the small 
openings in its foliage. The little images of the sun 
which form here and there on the ground will then have 



142 THE SUN, EARTH, AND MOON 

the form of the partially eclipsed sun. Soon the latter 
appears as the new moon, only instead of increasing, the 
crescent form grows thinner minute by minute. Even 
then, so well has the eye accommodated itself to the 
diminishing light, there may be little noticeable darkness 
until the crescent has grown very thin. If the observer 
has a telescope with a dark glass for viewing the sun, he 
will now have an excellent opportunity of seeing the 
mountains on the moon. The unbroken limb of the sun 
will keep its usual soft and uniform outline. But the 
inside of the crescent, the edge of which is formed by 
the surface of the moon, will be rough and jagged in 
outline. 

As the crescent is about to disappear the advancing 
mountains on the rugged surface of the moon will reach 
the sun's edge, leaving nothing of the latter but a row of 
broken fragments or points of light, shining between 
the hollows on the lunar surface. They last but a second 
or two and then vanish. 

Now is seen the glory of the spectacle. The sky is 
clear and the sun in mid-heaven, and yet no sun is visible. 
Where the latter ought to be the densely black globe of 
the moon hangs, as it were, in mid-air. It is surrounded 
by an effulgence radiating a saintly glory. This is the 
sun's corona, already mentioned in our chapter on the 
sun. Though bright enough to the unaided vision, it is 
seen to the best advantage with a telescope of very low 
magnifying power. Even a common opera glass may 
suffice. With a telescope of high power only a portion 
of the corona is visible, and thus the finest part of the 



ANCIENT ECLIPSES 143 

effect is lost. A common spy-glass, magnifying ten or 
twelve times, is better, so far as effect is concerned, than 
the largest telescope. Such an instrument will show not 
only the corona itself but the so-called "prominences" — 
fantastic cloud-like forms of rosy colour rising here and 
there, seemingly from the dark body of the moon. 

Ancient Eclipses 

It is remarkable that though the ancients were familiar 
with the fact of eclipses, and the more enlightened of 
them perfectly understood their causes, some even the 
laws of their recurrence, there are very few actual ac- 
counts of these phenomena in the writings of the ancient 
historians. The old Chinese annals now and then record 
the fact that an eclipse of the sun occurred at a certain 
time in some province or near some city of the empire. 
But no particulars are given. Quite recently the Assyri- 
ologists have deciphered from ancient tablets a statement 
that an eclipse of the sun was seen at Nineveh, B. C. 763, 
June 15. Our astronomical tables show that there actu- 
ally was a total eclipse of the sun on this day, during 
which the shadow passed a hundred miles or so north of 
Nineveh. 

Perhaps the most celebrated of the ancient eclipses, 
and the one that has given rise to most discussion, is that 
known as the eclipse of Thales. Its principal historical 
basis is a statement of Herodotus that in a battle between 
the Lydians and the Medes the day was suddenly turned 
Into night. The armies thereupon ceased battle and were 
more eager to come to terms of peace with each other. It 



144 THE SUN, EARTH, AND MOON 

is added that Thales, the Milesian, had predicted to the 
lonians this change of day, even the very year in which 
it should occur. Our astronomical tables show that there 
actually was a total eclipse of the sun in the year B. C. 
585, which was near enough to the time of the battle to 
be the one alluded to, but it is now known that the path 
of the shadow did not quite reach the seat of hostilities 
till after sunset. Some doubt therefore still rests on the 
subject. 

Prediction of Eclipses 

There is a curious law of the recurrence of eclipses 
which has been known from ancient times. It is based on 
the fact that the sun and moon return to nearly the same 
positions, relative to the node and perigee of the moon's 
orbit, after a period of six thousand five hundred and 
eighty-five days eight hours, or eighteen years and 
twelve days. This period is called the Saros. Eclipses 
of every sort repeat themselves at the end of a Saros. 
For example, the eclipse of May, 1900, may be regarded 
as a repetition of those which occurred in the years 1846, 
1864, and 1882. But when such an eclipse recurs it is 
not visible in the same part of the earth, because of the 
excess of eight hours in the period. During this eight 
hours the earth performs one third of a rotation on its 
axis, which brings a different region under the sun. Each 
eclipse is visible in a region about one third of the way 
round the world, or one hundred and twenty degrees of 
longitude, west of where it occurred before. Only after 
three periods will the recurrence be near the same region. 
But in the meantime the moon's line of motion will have 



THE SUN'S APPENDAGES 145 

changed so that the path of its shadow will pass farther 
north or south than before. 

There are two series of eclipses remarkable for the 
long duration of the total phase. To one of these the 
eclipse of 1868, hereafter mentioned, belongs. This re- 
curred in 1886, and will recur again in 1904. Unfortu- 
nately, at the first recurrence, the shadow was cast almost 
entirely on the Atlantic and Pacific Oceans, so that it 
was not favourable for observation by astronomers. That 
of 1904, September 9, will be yet more unfortunate for 
us, because the shadow will pass only over the Pacific 
Ocean. Possibly, however, it may touch some island 
where observations may be made. The recurrence of 
1922, September 1, will be visible in northern Australia, 
where the duration of totality will be about four minutes. 

To the other and yet more remarkable series belonged 
the eclipse of May 7, 1883, and that of May 11, 1901. 
At the successive recurrences of this eclipse the duration 
of totality will be longer and longer through the twenti- 
eth century. In 1937, 1955, and 1973 it will exceed 
seven minutes, so that so far as duration is concerned, our 
successors will see eclipses more remarkable than any 
their ancestors have enjoyed for many centuries. 

The Sun's Appendages 

About 1863-64 the spectroscope began to be applied 
to researches on the heavenly bodies. Mr. (now Sir 
William) Huggins, of London, was a pioneer in observ- 
ing the spectra of the stars and nebulse. For several 
years it did not seem that much was to be learned in this 



146 THE SUN, EARTH, AND MOON 

way about the sun. The year 1868 at length arrived. 
On August eighteenth there was to be a remarkable total 
eclipse of the sun, visible in India. The shadow was one 
hundred and forty miles broad ; the duration of the total 
phase was more than six minutes. The French sent Mr. 
Janssen, one of their leading spectroscopists, to observe 
the eclipse in India and see what he could find out. Won- 
derful was his report. The red prominences which had 
perplexed scientists for two centuries were found to be 
immense masses of glowing hydrogen, rising here and 
there from various parts of the sun, of a size compared 
with which our earth was a mere speck. This was not 
all. After the sunlight reappeared, Janssen began to 
watch these objects in his spectroscope. He followed 
them as more and more of the sun came out, and con- 
tinued to see them until after the eclipse was over. They 
could be observed at any time when the air was sufficiently 
clear and the sun high in the sky. 

By a singular coincidence tliis same discovery was 
made independently in London without any eclipse. Mr. 
J. Norman Lockyer was then rising into prominence as 
an enthusiastic worker with the spectroscope. It oc- 
curred independently to him and to Mr. Huggins that 
the heat in the neighbourhood of the sun was so intense 
that any matter that existed there would probably take 
the form of a gas shining by its own light. Both of 
these investigators endeavoured to get a sight of the 
prominences in this way; but it was not until October 
twentieth, two months after the Indian eclipse, that Mr. 
Lockyer succeeded in having an instrument of sufficient 



THE SUN'S APPENDAGES 147 

power completed. Then, at the first opportunity, he 
found that he could see the prominences without an 
eclipse ! 

At that time communication with India was by mail, 
so that for the news of Mr. Janssen's discovery astrono- 
mers had to wait until a ship arrived. By a singular 
coincidence his report and Mr. Lockyer's communication 
announcing his own discovery reached the French Acad- 
emy of Sciences at the same meeting. This eminent body, 
with pardonable enthusiasm, caused a medal to be struck 
in commemoration of the new method of research, in 
which the profiles of Lockyer and Janssen appeared to- 
gether as co-discoverers. Since that time the promi- 
nences are regularly mapped out from day to da}^ by 
spectroscopic observers in various parts of the world. 

The greatest beauty of a total eclipse is due to the 
sun's corona. The exact nature of this appendage is 
still in doubt. Indeed, until photography was called to 
the aid of the astronomer its structure was unknown. It 
was described by observers simply as a soft light sur- 
rounding the sun ; but when it is photographed and care- 
fully examined it is found to be of a radial, hairy 
structure wliich the reader can easily see from the fron- 
tispiece of the book. It extends out farthest in the 
direction of the sun's equator and least at the poles. The 
rays which chance to be exactl}^ at the poles go straight 
out from the sun. But those on each side are found to 
curve toward the equator, while farther from the equator 
they are lost in the more powerful effulgence going out 
from the region of the solar spots. Near the poles the 



148 THE SUN, EARTH, AND MOON 

forms are remarkably like those which iron filings assume 
when scattered on paper above a magnet. It is therefore 
a question whether there is not here something in the na- 
ture of a magnetic force. But in the region called the 
sun's equator this analogy ceases to hold. In describing 
the sun we mentioned the much greater activity in the 
regions of greater spottedness than elsewhere. It now 
seems as if the forces which throw out the corona are 
also greatest where the sun's activity is greatest. 

The probability now seems to be that the corona is 
composed of matter thrown up from the sun, and kept 
from falling back again by the repulsion of the solar 
rays, and that it bears a certain resemblance to the tail 
of a comet. 

A very important question is whether the corona shines 
mostly by reflected light, or by its own light, due to the 
high temperature which it must have so near the sun. 
No doubt its light arises from both sources, but it is not 
yet known in what proportion. The fact is that its 
spectrum shows some bright lines. These can be due 
only to the light of the matter itself. Some observers 
have supposed that they also saw dark lines in the spec- 
trum. This, however, has not been proved. On the whole 
the probability seems to be that the corona shines mostly 
by its own light. 



PART IV 
THE PLANETS AND THEIR SATELLITES 



I . 

Orbits and Aspects of the Planets 

The orbits in which the planets revolve around their 
central luminary are in strictness ellipses, or slightly 
flattened circles. But the flattening is so slight that the 
eye would not notice it without measurement. The sun 
is not in the centre of the ellipse but in a focus, which 
in some cases is displaced from the centre by an amount 
that the eye can readily perceive. This displacement 
measures the eccentricity of the ellipse, which is much 
greater than the flattening. For example, in the case 
of Mercury, which moves in a very eccentric orbit, the 
flattening is only one fiftieth; that is, if we represent 
the greatest diameter of the orbit by fifty, the least 
diameter will be forty-nine. But the distance of the 
sun from the centre of the orbit is ten on the same 
scale. 

To show this we give a diagram of the orbits of the 
inner group of planets showing quite nearly their forms 
and respective locations. A simple glance will show that 
the orbits are much nearer together at some points than 
at others. 

In explaining the various aspects and motions, real 
and apparent, of the planets a number of technical ex- 
pressions are used which we shall explain. 

Inferior planets are those whose orbits lie within the 



152 PLANETS AND THEIR SATELLITES 

orbit of the earth. This class comprises only Mercury 
and Venus. 

Superior planets are those whose orbits lie without 
that of the earth. These comprise iMars, the minor 
planets or asteroids, and all four of the outer group of 
major planets. 

When a planet seems to us to pass by the sun, and so 



MARS 




Fig. 27. — Orbits of the Four Inner Planets. 

is seen as if alongside of it, it is said to be in conjunction 
with the sun. 

An inferior conjunction is one in which the planet is 
between us and the sun. 

A superior conjunction is one in w^hich the planet is 
beyond the sun. 



ORBITS AND ASPECTS OF PLANETS 153 

A little consideration will show that a superior planet 
can never be in inferior conjuction, but an inferior planet 
has both kinds of conjunction. 

A planet is said to be in opposition when it is in the 
opposite direction from the sun. It then rises at sunset, 
and vice versa. Of course, an inferior planet cai> never 
be in opposition. 

The perilielio7i of an orbit is that point of it which is 
nearest the sun ; the aphelion its most distant point from 
the sun. 

As the inferior planets, Mercury and Venus, perform 
their revolutions they seem to us to swing from one side 
of the sun to the other. Their apparent distance from 
the sun at any time is called their elongation. 

The greatest elongation of Mercury is generally about 
twenty-five degrees, being sometimes more and sometimes 
less, owing to the great eccentricity of the orbit of this 
planet. The greatest elongation of Venus is almost 
forty -five degrees. 

When the elongation of one of these planets is east 
from the sun we may see it in the west after sunset; 
when west we may see it in the east in the morning sky. 
As neither of them ever wanders from the sun farther 
than the distances we have stated, it follows that a planet 
seen in the east in the evening, or in the west in the morn- 
ing, cannot be either Mercury or Venus. 

No two orbits of the planets lie exactly in the same 
plane. That is, if we regard any one orbit as horizontal, 
all the others will be tipped by small amounts toward one 
side or the other. Astronomers find it convenient to take 



154 PLANETS AND THEIR SATELLITES 

the orbit of the earth, or the ediptic, as the horizontal or 
standard one. As each orbit is centred on the sun it 
will have two opposite points which lie on the same hori- 
zontal plane as the earth's orbit. More exactly, these 
are the points at which the orbit intersects the plane of 
the ecliptic. They are called nodes. 

The angle by which an orbit is tipped from the plane 
of the ecliptic is called its inclination. The orbit of Mer- 
cury has the greatest inclination, more than 6°. Tiie 
orbit of Venus is inclined 3° S4' ; those of all the superior 
planets less, ranging from 0° 46' in the case of Uranus 
to 2° 30' in the case of Saturn. 

Distances of the Planets 

Leaving out Neptune, the distances of the planets 
follow very closely a rule known as B ode's Law, after the 
astronomer who first pointed it out. It is this : Take the 
numbers 0, 3, 6, 12, etc., doubling each as we go along. 
Then add 4 to each number, and we shall hit very nearly 
on the scale of distances of all the planets except Nep- 
tune, thus : 



Mercury, 4- 4 = 4 


actual distance 4 


Venus, 3 + 4 = 7 


" 


7 


Earth, 6 + 4 = 10 




10 


Mars, 13 + 4 = 16 




15 


Asteroids, 34 + 4 = 38 




' 30 to 40 


Jupiter, 48 + 4 = 53 




53 


Saturn, 96 + 4 = 100 




95 


Uranus, 193 + 4 = 196 




' 193 


Neptune, 384 + 4 = 388 




' 300 



On these actual distances we remark that astronomers do 



V 



KEPLER'S LAWS 155 

not use miles or other terrestrial measures to express 
distances between the heavenly bodies, for two reasons. 
In the first place, they are too short; to use them w^ould 
be like stating the distance between two cities in centi- 
metres. In the next place, distances in the heavens can- 
not be fixed with the necessary exactness in our measures, 
whereas, if we take the sun's distance from the earth as 
the unit of measure, we can determine other distances 
between the planets with great precision in terms of this 
measure. So, to get the distances of the planets from 
the sun in astronomical measure, we have to divide the 
last numbers of the preceding table by ten, or insert a 
decimal point before the last figure of each. 

We have not in this table distracted the attention of 
the reader by using unnecessary decimals. Actually, the 
distance of Mercury is 0.387, etc. ; we have simply called 
it 0.4 and multiplied it by 10 to get the proportion for 
comparing with Bode's Law. 

Kepler^s Laws 

Th€ motions of the planets in their orbits take place 
in accordance with certain laws laid down by Kepler, 
and therefore known as Kepler's laws. The first of these 
has already been mentioned ; the orbits of the planets are 
ellipses, of which the sun is in one focus. 

The second law is that the nearer the planet is to the 
sun the faster it moves. With more mathematical exact- 
ness, the areas swept over by the line joining the planet 
and sun in equal times are all equal. 

The third law is that the cubes of the mean distances 



156 PLANETS AND THEIR SATELLITES 

of the planets from the sun are proportional to the 
squares of their times of revolution. This law requires 
some illustration. Suppose one planet to be four times 
as far from the sun as another. It will then be eight 
times as long going around it. This number is reached 
by taking the cube of four, which is sixty-four, and then 
extracting the square root, which is eight. 

The unit of measure which the astronomer uses to ex- 
press distances in the solar system being the mean dis- 
tance of the earth from the sun, it follows that the mean 
distances of the inferior planets will be decimal fractions, 
as we have just shown, while those of the outer ones will 
vary from 1.5 in the case of Mars to 30 in the case of 
Neptune. If we take the cubes of all these distances and 
extract their square roots we shall have the times of the 
revolution of the planets, expressed in years. 

It will be seen that the outer planets are longer in 
getting around their orbits, not only because they have 
farther to go, but because they actually move more 
slowly. If, as in the case first supposed, the outer planet 
is four times as far from the sun, it will move only half 
as fast. This is why it takes eight times as long to get 
around. The speed of the earth in its orbit is about 
18.6 miles per second. But that of Neptune is only 
about 3.5 miles per second, although it has thirty times 
as far to go. This is why it takes more than one hun- 
dred and sixty years to complete a revolution. 



II 

The Planet Mercury 

To set forth what is known of the major planets we 
shall take them up in the order of their distance from the 
sun. The first planet reached will then be Mercury. It 
is not only the nearest planet to the sun, but much the 
smallest of the eight; so small, indeed, that, but for its 
situation, it would hardly be called a major planet. Its 
diameter is about two fifths greater than that of the 
moon, but, the volumes of bodies being proportional to 
the cubes of their diameters, it has about three times the 
volume of the moon. 

It has far the most eccentric orbit of all the major 
-planets, though, in this respect, it is exceeded by some 
of the minor planets to be hereafter described. In conse- 
quence, its distance from the sun varies between wide 
limits. At perihelion it is less than twenty-nine millions 
of miles from the sun ; at aphelion it goes out to a distance 
of more than forty-three millions of miles. It performs 
its revolution around the sun in a little less than three 
months ; to speak more exactly, in eighty-eight days. It 
tlierefore makes more than four revolutions in a year. 

Performing more than four revolutions around the 
sun while the earth is performing one, we readily see 
that it must pass conjunction with the sun at certain 
regular though somewhat unequal intervals. To show 



158 PLANETS AND THEIR SATELLITES 

the exact nature of its apparent motion let the inner 
circle of the diagram represent the orbit of Mercury and 
the outer one that of the earth. When the earth is at E, 
and Mercury at M, the latter is in inferior conjunction 
with the sun. At the end of three months it will have re- 
turned to the point M, but it will not yet be in conjunc- 



/ 



r 



\ 
\ 
\ 



7 .,„., X 



i or t. \ 



JE JN_F_ERLOR /m M^ 

T CON J UNCTION T ^Mt 



I V 



\ \ 






\ 



\ 






V 



Fig. 28. — Conjunctions of Mercury with the Sun. 

tion, because, in the meantime, the earth has moved for- 
ward in its orbit. When the earth reaches a certain point 
r, Mercury wiU have reached the point N and will again 
be in inferior conjunction. This revolution from one in- 
ferior conjunction to another is called the synodic revolu- 
tion of the planet. In the case of Mercury this is some- 
what less than one third more than the time of actual 



THE APPEARANCE OF MERCURY 159 

revolution ; that is to say, the arc MN is a little less than 
one third of the circle. 

Now suppose that when the earth is at E, Mercury, 
instead of being at M is near the highest point A of the 
orbit as represented in the figure. It will then be at 
its- greatest apparent distance from the sun as we see 
it from the earth; or, in technical language, at its 
greatest east elongation. Being east of the sun it will 

,, e^^^-" -4 • i 

EARTH .,>"-' --'-"" /■ vSUN ^\ 

<*^7 ^r Si 



Fig. 29. — Elongations of Mercury. 

then set after the sun, by a time generally between 
an hour and a quarter and an hour and a half. This 
is the most convenient time for seeing it. If the sky 
is clear, it will readily be seen in the twilight from half 
an hour to an hour after sunset. At the opposite elonga- 
tion, near C, it is west of the sun ; then it rises before the 
sun and may be seen in the morning twilight. 

The Surface and Rotation of Mercury 

The best time to make a telescopic study of Mercury 
is late in the afternoon, when it is near east elongation. 



160 PLANETS AND THEIR SATELLITES 

or shortly after sunrise, if it rises before the sun. Sup- 
posing it east of the sun, it will probably be visible in 
the telescope at any time after noon, but the air is gen- 
erally disturbed by the sun's rays so that it is hardly 
possible to make a good observation at that time. Late 
in the afternoon the air grows steadier, so that the planet 
can be better observed. But, after sunset, the planet is 
seen through a continually increasing extent of atmos- 
phere, so that the seeming disturbance again begins to 
increase. Owing to these circumstances it is the most 
difficult of all the planets to study in a satisfactory way, 
and observers differ very much as to what can be seen on 
its surface. 

The first observer who thought he could see any fea- 
tures on the surface of this planet was Schroter, a Ger- 
man. When Mercury presented the form of a crescent 
he fancied that its south horn seemed blunted at inter- 
vals. He attributed this to the shadow of a lofty moun- 
tain ; and by observing the intervals between the blunted 
appearance he concluded that the planet revolved on its 
axis in twenty-four hours and five minutes. But Sir 
William Herschel, who observed at the same time with 
much more powerful instruments, could not see anything 
of the kind. 

Until quite recently nearly all observers agreed with 
Herschel that no time of rotation could be certainly de- 
termined. But a few years since, Schiaparelli, observing 
with a fine telescope in the beautiful sky of northern 
Italy, noticed that the aspect of the planet seemed un- 
changed day after day. He was thus led to the conclu- 



THE PHASES OF MERCURY 161 



sion that it always presents the same face to the sun, 
as the moon presents the same face to the earth. This 
view was shared by Mr. Lowell, observing at the Flag- 
staff Observatory. But the observation is too difficult 
to permit us to regard the fact as established. All that 
a conservative astronomer would be willing to say is 
that as yet w^e know nothing of the revolution of Mercury 
on its axis. 

Drawings showing the face of Mercury have been 
made by several astronomers. As it is seen under all 
ordinary conditions no special features are well marked. 
Very different is the case at the Lowell Observatory in 
Flagstaff, Ariz. The most singular feature of its sur- 
face in the latter picture consists in the dark lines which 
cross it. These have not been seen by other observers, 
and, until they are established by independent evidence, 
astronomers will be sceptical as to their reality. The 
reason of this will be stated later in connection with the 
planet Mars. 

Owing to the various positions of Mercury relative to 
the sun it presents phases like those of the moon. These 
depend upon the relation of the dark and the illuminated 
hemispheres relative to the direction in which we see the 
planet. The hemisphere which is turned away from the 
sun, being in darkness, is always invisible to us. At 
superior conjunction the illuminated hemisphere is turned 
toward us and the planet seems round, like a full moon. 
As it moves from east elongation to inferior conjunction, 
more and more of the dark hemisphere is turned toward 
us, and less and less of the illuminated one. But this 



162 PLANETS AND THEIR SATELLITES 

disadvantage is counterbalanced by the fact that the 
planet continually comes nearer during the interval, so 
that we get a better view of whatever portion of the 
illuminated hemisphere may be visible to us. Its appar- 
ent form and size at different times during its synodic 
revolution go through a series of changes similar to those 
shown in the next chapter in the case of Venus. 

The question whether Mercury has an atmosphere is 
also one on which opinions differ, the prevailing opinion 
being in the negative. It seems quite certain that, if it 
has one, it is too rare to reflect the light of the sun. 

Transits of Mercury 

It will be readily seen that, if an inferior planet re- 
volved around the sun in the same plane as the earth, we 
should see it pass over the sun's disk at every inferior 
conjunction. But no two planets revolve in the same 
plane. Of all the major planets the orbit of Mercury 
has the largest inclination to that of the earth. In con- 
sequence, when in inferior conjunction, it commonly 
passes a greater or less distance to the north or to the 
south of the sun. If, howcA^er, it chances to be near one 
of its nodes at the time in question, we shall see it as 
a black spot passing across the sun's disk. This phe- 
nomenon is called a transit of Mercury. Such transits 
occur at intervals ranging between three and thirteen 
years. They are observed with much interest by as- 
tronomers because it is possible to determine with great 
precision the time at which the planet enters upon the 
solar disk, and leaves it again. Knowing these times, 



TRANSITS OF MERCURY 163 

valuable information is afforded respecting the exact law 
of motion of the planet. 

The first observation of a transit of Mercury was made 
by Gassendi on November 7, 1631. His observation is 
not, however, of any scientific value at the present time, 
owing to the imperfection of his instruments. A some- 
what better but not good observation was made by Hal- 
ley, of England, in 1677, during a visit to the island of 
St. Helena. Since that time the transits have been ob- 
served with a fair degree of regularity. The following, 
table shows the transits that will be visible during the 
next fifty years, with the regions of the earth in which 
each may be seen : 

1907, November 14, visible in Europe and eastern United 

States. 
1914, November 7, visible in the same regions. 
1924, May 7, the beginning will be visible on the Pacific 

coast, but the whole transit only on the Pacific 

Ocean and in eastern Asia. 
1927, November 9, visible in Asia and eastern Europe. 
1937, May 11, Mercury will graze the south limb of 

the sun. The phenomenon will be visible in Europe, 

but will occur before the sun rises in America. 
1940, November 10, visible in the Western and Pacific 

States. 
1953, November 14, visible throughout the United 

States. 

Observations of transits of Mercury since 1677 have 
brought out one of the most perplexing facts of astron- 



164 PLANETS AND THEIR SATELLITES 

omy. The orbit of this planet is found to be slowly 
changing its position, its perihelion moving forward by 
about forty-three seconds per century farther than it 
ought to move in consequence of the attraction of all the 
known planets. This deviation was discovered in 1845 
by Le Verrier, celebrated as having computed the posi- 
tion of Neptune before it had ever been recognised in 
the telescope. He attributed it to the attraction of a 
planet, or group of planets, between Mercury and the 
sun. His announcement set people to looking for the 
supposed planet. About 1860, a Dr. Lescarbault, a 
country physician of France, who possessed a small tele- 
scope, thought he had seen this planet passing over the 
disk of the sun. But it was soon proved that he must 
have been mistaken. Another more experienced astron- 
omer, who was looking at the sun on the same day, failed 
to see anything except an ordinary spot. It was prob- 
ably this which misled the physician-astronomer. Now, 
for foiH:y years, the sun has been carefully scrutinised 
and photographed from day to day at several stations 
without anything of the sort being seen. 

Still, it is possible that little planets so minute as to 
escape detection in passing over the sun's disk may re- 
volve in the region in question. If so, their light would 
be completely obscured by that of the sky, so that they 
might not ordinarily be visible. But there is still a 
chance that, during a total eclipse of the sun, when the 
light is cut off from the sky, they could be seen. Ob- 
servers have, from time to time, looked for them during 
total eclipses. In one instance something of the sort was 



INTRAMERCURIAL PLANETS 165 

supposed to be found. During the eclipse of 1878, 
Professor Watson, of Ann Arbor, and Professor Lewis 
Swift, both able and experienced observers, thought that 
they had detected some such bodies. But critical exam- 
ination left no doubt that what Watson saw was a pair 
of fixed stars which had always been in that place. How 
it was with the observations of Professor Swift has never 
been certainly ascertained, because he was not able to lay 
down the position with such certainty that positive con- 
clusions could be drawn. 

Notwithstanding such failures, observers have repeated 
the search during several of the principal total eclipses. 
The writer did so during the eclipse of 1869, and again 
during that of 1878, the search being made with a small 
telescope. In recent times the powerful agency of 
photography has been invoked by Professors Pickering 
and Campbell during the eclipses of 1900 and 1901. 
Campbell's results during the latter eclipse were the most 
decisive yet reached. With his photographic telescope 
some fifty stars were photographed, some as faint as the 
eighth magnitude, but they were all found to be known 
objects. It therefore seems certain that there can be no 
intramercurial much brighter than the eighth magnitude. 
It would take hundreds of thousands of such planets as 
this to produce the observed motion of Mercury. So great 
a number of these bodies would produce a far brighter 
illumination of the sky than any that we see. The result 
therefore seems to be conclusive against the view that the 
motion of the perihelion of Mercury'- can be produced by 
intramercurial planets. In addition to all these difHcul- 



166 PLANETS AND THEIR SATELLITES 

ties in supposing the planet to exist we have the difficulty 
that, if it did exist, it would produce a similar though 
smaller change in the position of the nodes of either Mer- 
cury or Venus, or both. 

Altogether, the evidence seems conclusive against the 
reality of any bodies whose attraction could produce the 
observed deviation, which still remains unexplained. The 
most recent supposition on the subject is that the force 
of gravitation deviates slightly from the law of the in- 
verse square. But this requires farther investigation. 



Ill 

The Planet Venus 

Or all the star-like objects in the heavens the planet 
Venus is the most brilliant. The sun and moon are the 
only heavenly bodies outshining it. In a clear and moon- 
less evening it may be seen to cast a shadow. If an 
observer knows exactly where to look for it, and has a 
well-focused eye, it can be seen in the daytime when near 
the meridian, provided that the sun is not in its immediate 
neighbourhood. When it is east of the sun it may be seen 
in the west, faintly before sunset and growing continually 
brighter as the light diminishes. When west of the sun 
it rises in the morning before the sun, and may then be 
seen in the east. Under these circumstances it has been 
called the evening and morning star respectively. The 
ancients called it Hesperus when an evening star, and 
Phosphorus when a morning star. It is said that, in the 
early history of our race, Hesperus and Phosphorus were 
not known to be the same body. 

If Venus is examined with the telescope, even one of 
low power, it will be seen to exhibit phases like those of 
the moon. This fact was ascertained by Galileo when 
he first directed his telescope toward the planet, and af- 
forded him strong evidence of the truth of the Coperni- 
can System. In accordance with a custom of the time he 
published this discovery in the form of an anagram — a 



168 PLANETS AND THEIR SATELLITES 

collection of letters which, when subsequently put to- 
gether would state the discovery. Translated Into Eng- 
lish the anagram read, "The mother of the loves emulates 
the phases of Cynthia." 

What we have said of the synodic motion of Mercury 
applies in principle to Venus, and need not therefore be 
repeated. In the following cut the apparent size of the 
planet is shown in various parts of its synodic orbit. As 
the planet passes from superior to inferior conjunction 
its globe continually grows larger in apparent size. 




Fig. 30. — Phases of Venus in Different Points of its Orbit. 

though we cannot see its entire outline. But the fraction 
of the disk illuminated continually becomes smaller, first 
having the shape of a half moon, and then the shape of 
a crescent, which grows thinner and thinner up to the 
time of inferior conjunction. In the latter position the 
dark hemisphere is turned toward us and the planet Is 
invisible. Venus Is at its greatest brightness about half- 
way between Inferior conjunction and greatest elonga- 
tion. It then sets about two hours after the sun, if east 
of it, and rises about two hours before the sun, if west 
of It. 



ROTATION OF VENUS 169 

Rotation of Venus 

The question of the rotation of Venus has interested 
astronomers and the pubhc ever since the time of Gahleo. 
But the difficulty of learning anything certain on the 
subject is very great, owing to the peculiar glare of the 
planet. When seen through a telescope no sharp and 
well-defined markings are visible. Instead of this there 
is a glare on the surface, varying by gentle gradations 
from one region to another, as if we were looking upon 
a globe of polished but slightly tarnished metal. Never- 
theless, various observers have supposed that they could 
distinguish bright or dark spots. As far back as 1667 
Cassini concluded from these seeming spots that the 
planet revolved on its axis in a little less than twenty- 
four hours. During the next century Blanchini, an 
Italian observer, published an extensive treatise on the 
subject, illustrated with many drawings of the planet. 
His conclusion w^as that Venus required more than twenty- 
four da3^s to revolve on its axis. Cassini, the son, de- 
fended his father's conclusion by claiming that the planet 
had always made one revolution and a little more between 
the times of Blanchini's observations on successive even- 
ings. Thus the Italian astronomer would naturally see 
the spots on successive evenings a little farther advanced, 
and estimated the motion by this advance, not being aware 
that a whole revolution had been made during the interval. 
At the end of twenty-four days the same hemisphere of 
the planet would be presented to the earth as before, the 
number of revolutions in the meantime being twenty-five. 



170 PLANETS AND THEIR SATELLITES 

Schroter tried to decide the question for Venus in the 
same way that he supposed himself to have decided it for 
Mercury. He directed his attention especially to the fine 
sharp horns of the crescent, when the planet was nearly 
between the earth and the sun. At certain intervals he 
supposed one of them to be a little blunted. Ascribing 
this appearance to the shadow of a high mountain, he 
concluded that the time of rotation was twenty-three 
hours twenty-one minutes. 

From the time of Schroter no one professed to throw 
any more light on the question until 183^. Then De 
Vico, of Rome, announced that he had rediscovered the 
markings found by Blanchini. He concluded that the 
planet rotated in twenty-three hours twenty-one minutes, 
in agreement with Schroter's result. 

This close agreement between the results of observa- 
tions by four distinguished observers led to the very gen- 
eral acceptance of twenty-three hours twenty-one minutes 
as the time of rotation of the planet. But there was much 
to be said on the other side. The great Herschel, with 
the most powerful telescopes that had ever been made, was 
never able to make out any permanent markings on Venus. 
If anything like a spot appeared, it varied and disap- 
peared again so rapidly that no evidence of rotation could 
be afforded by it. This negative result has always been 
reached by the large majority of observers. 

But a new and surprising theory has been recently put 
forth by Schiaparelli, and maintained by Lowell. This is 
that Venus rotates on its axis in the same period that it 
revolves around the sun; in other words both Mercury 



ROTATION OF VENUS 171 

and A^enus always present the same face to the sun, as the 
moon presents the same face to the earth. Schiaparelh 
reached this conclusion by noticing that a number of ex- 
ceedingly faint spots could be seen on the southern hemi- 
sphere of Venus for several days in succession in the 
same position day after dayo He could observe the planet 
through several hours on each day, and the constancy 
of the spots precluded the idea that the planet made one 
rotation and a little more in the course of a day. Lowell 
was led to the same conclusion by careful study of the 
planet at his Arizona observatory. 

The latest conclusion has been reached by the spectro- 
scope. We have already explained how, with this instru- 
ment, it can be determined whether a heavenly body is 
moving toward us or from us. The principle applies 
to a planet which we see by the reflected light of the sun 
as well as to a star. Hence, if Venus rotates, one part 
of its disk will be moving toward us, and the other from 
us. By comparing the dark lines of the spectrum shown 
by the two edges of the disk of Venus it can then be de- 
termined how various points of the disk are moving with 
respect to the earth. It was thus found by Belopolsky 
that the planet was affected by a quite rapid rotation. 
The observation is so difficult, and the displacement of 
the lines so small, that it was not possible to state a very 
certain result, although the general fact was made very 
probable. On the whole we must regard this conclusion 
as the most likely that has yet been reached, although it 
is at variance with the observations of Schiaparelli, as 
well as those of the Lowell Observatory. But the spectro- 



172 PLANETS AND THEIR SATELLITES 

scopic observations have not yet been made with sufficient 
precision to teach us the exact time of revolution. Re- 
cent discoveries as to the nature of the atmosphere of 
Venus make it almost certain that all the observers who 
supposed that they saw markings on the planet were 
mistaken. 

Atmosphere of Venus 

It is now well established that Venus is stirrounded 
by an atmosphere which is probably denser than that of 
the earth. This was shown in a remarkable and interest- 




FiG. 31. — I^j/'ed of the Atmosphere of Venus during the Transit of 1882. 



ing way during the transit of Venus over the sun's disk 
in 1882, which was observed by the writer at the Cape 
of Good Hope. When the planet was a little more than 
halfway on the disk, its outer edge appeared illuminated, 
as shown on the figure. This illumination, however, did 
not commence at the middle point of the arc, as it 



ATMOSPHERE OF VENUS 173 

should have done had it been caused by regular refrac- 
tion, but commenced at a point quite near one end of the 
arc. This appearance was explained by Russell, of 
Princeton, who showed that the atmosphere is so full 
of vapour that we cannot see the light of the sun by 
direct refraction through it. What we see is an illu- 
minated stratum of clouds or vapour floating in an at- 
mosphere. Such being the case, it is not at all likely that 
astronomers on the earth can ever see the solid body of 
the planet through these clouds. Hence the supposed 
spots could only have been temporary clouds, continually 
changing. 

To illustrate the illusions to which the sight of even 
good observers may be subject, we may mention the fact 
that several such observers have supposed the whole hemi- 
sphere of Venus to be visible when the planet was near 
inferior conjunction. It then had the appearance fa- 
miliarly known as "the new moon in the old moon's 
arms," with which everyone who observes our satellite 
when a narrow crescent is familiar. In the case of the 
moon it is well known that we thus see the dark hemi- 
sphere by the light reflected from the earth. But in 
the case of Venus there is no possibility of a sufficient 
reflection of light from the earth, or any other body. 
The appearance has sometimes been explained by a possi- 
ble phosphorescence covering the whole hemisphere of 
Venus. But it is more likely due to an optical illusion. 
It has generally been seen in the daytime, when the 
sky is brightly illuminated, and when any faint light 
like that of phosphorescence would be completely in- 



174 PLANETS AND THEIR SATELLITES 

visible. To whatever we might attribute the hght, it 
ought to be seen far better after the end of twihght in the 
evening than during the daytime. The fact that it is not 
seen then seems to be conclusive against its reality. 

The appearance illustrates a well-known psychological 
law, that the imagination is apt to put in what it is ac- 
customed to see, even when the object is not there. We 
are so accustomed to the appearance on the moon that 
when we look at ^"enus the similarity of the general phe- 
nomena leads us to make this supposed familiar addition 
to it. 

Has Venus a Satellite? 

During the past two centuries several observers have 
from time to time thought that they saw a satellite of 
Venus. Countless observers, with good telescopes, have 
seen nothing of the sort. We may safely say that Venus 
has no satellite visible in the most powerful telescopes 
of our time. Quite likely these supposed satellites were 
seeming objects quite familiar to astronomers under the 
name of "ghosts." These are sometimes seen when a 
telescope is pointed at a bright object, and are due to a 
double reflection of light in the lenses either of the object- 
glass or the eyepiece. 

A few years ago the writer received a letter from the 
owner of a very large telescope in England stating that, 
by great care, he could see a very faint, round, and well- 
defined aureole of light around the planet Mars. He 
desired to know whether the object could be real, or how 
the appearance was to be explained. In reply, he was 
informed that such an appearance would be produced 



TRANSITS OF VENUS 175 

by the double reflection of light between the two inner 
lenses of the object-glass, provided their curvatures were 
nearly, but not exactly the same. It was suggested that 
he point the telescope at Sirius and see if a similar ap- 
pearance did not surround the star. He probably found 
that such was the case. 

Transits of Venus 

The transits of Venus across the sun's disk are among 
the rarest phenomena of astronomy, as they occur, on 
the average, only once in sixty years. For many cen- 
turies past and to come there will be a regular cycle, 
bringing about four transits in two hundred and forty- 
three years. The intervals between the transits are one 
hundred and five and a half years, eight years, one hun- 
dred and twenty-one and a half years, eight years ; then 
one hundred and five and a half years again, and so on. 
The dates of the last six transits and the two next to 
come are as follows: 

1631, December 7, 1874, December 9, 

1639, December 4, 188S, December 6, 

1761, June 5, 2004, June 8, 

1769, June 3, 2012, June 6. 

It will be seen that no person now living is likely to see 
this phenomenon, as the next transit does not occur until 
2004. Yet, the time when Venus will appear upon the 
disk on June 8 of that year can now be predicted for any 
point on the earth's surface, within a minute or two. 



176 PLANETS AND THEIR SATELLITES 

The interest which has attached to these transits dur- 
ing the past century arose from the fact that they were 
supposed to afford the best method of determining the 
distance of the sun from the earth. This fact and the 
rarit}^ of the phenomenon led to the last four transits 
being observed on a large scale. In 1761, and again in 
1769, the leading maritime nations sent observers to 
various parts of the world to note the exact time at 
which the planet entered upon and left the sun's disk. In 
1874 and 1882, expeditions were fitted up on a large 
scale by the United States, Great Britain, France, and 
Germany. On the first of these occasions American par- 
ties occupied stations in China, Japan, and eastern 
Siberia on the north, and in Australia, New Zealand, 
Chatham Island, and Kerguelen Island in the south. In 
1882 it was not necessary to send out so many expedi- 
tions, because the transit was visible in this country. In 
the southern hemisphere stations were occupied at the 
Cape of Good Hope and other points. The observations 
made by these expeditions proved of great value in de- 
termining the future motions of Venus, but it was found 
that other methods of determining the distance of the 
sun would lead to a more certain result. 



The Planet Mars 

More public interest has in recent years been con- 
centrated on the planet Mars than on any other. Its 
resemblance to our earth, its supposed canals, oceans, 
climate, snowfall, etc., have all tended to interest us in 
its possible inhabitants. At the risk of disappointing 
those readers who would like to see certain proof that our 
neighbouring world is peopled with rational beings, I 
shall endeavour to set forth what is actually known on 
the subject, distinguishing it from the great mass of illu- 
sion and baseless speculation which has crept into popular 
journals during the past twenty years. 

We begin with some particulars which will be useful 
in recognising the planet. Its period of revolution is 
six hundred and eighty-seven days, or forty-three days 
less than two years. If the period were exactly two years, 
it would make one revolution while the earth made two, 
and we should see the planet in opposition at regular in- 
tervals of two years. But, as it moves a little faster than 
this, it takes the earth from one to two months to catch 
up with it, so that the oppositions occur at intervals of 
two years and one or two months. This excess of one or 
two months makes up a whole year after eight opposi- 
tions ; consequently, at the end of about seventeen years, 
INIars will again be in opposition at the same time of the 



178 PLANETS AND THEIR SATELLITES 

year, and near the same point of its orbit, as before. In 
this period the earth will have made seventeen revolutions 
and Mars nine. 

The difference of a month or so in the interval be- 
tween oppositions is due to the great eccentricity of the 
orbit, which is larger than that of any other major 
planet except Mercury. Its value is 0.093, or nearly 
one tenth. Hence, when in perihelion, it is nearly one 
tenth nearer the sun than its mean distance, and when 
in aphelion nearly one tenth farther. Its distance from 
the earth at opposition will be different by the same 
amount, measured in miles, and hence in a much larger 
proportion to the distance itself. If opposition occurs 
when the planet is near perihelion, the distance from 
earth is about forty-three million miles; but if near 
the aphelion, about sixty million miles. The result of 
this is that, at a perihelion opposition, which can occur 
only in September, the planet will appear more than 
three times as bright as at an aphelion opposition, occur- 
ing in February or March. An opposition occurred 
near the end of March, 1903 ; the next following early 
in May, 1905. We shall then have oppositions near the 
end of June, 1907, and in August, 1909, which will be 
quite near to perihelion. 

Mars, when near opposition, is easily recognised by its 
brilliancy, and by the reddish colour of its light, which is 
very different from that of most of the stars. It is 
curious that a telescopic view of the planet does not give 
so strong an impression of red light as does the naked eye 
view. 



SURFACE AND ROTATION OF MARS 179 

The Surface and Rotation of Mars 

The great Hujgens, who flourished between 1650 and 
1700, studying Mars with the telescope, was the first one 
to recognise the variegated character of its surface, and 
to make a drawing of the appearance which it presented. 
The features delineated by Huygens can be recognised 
and identified to this day. By watching them it was easy 
to see that the planet rotated on its axis in a little more 
than one of our days (24h. 37m.). 

This time of rotation is the only definite and certain 
one among all the planets besides the earth. For two 
hundred j^ears Mars has rotated at exactly this rate, and 
there is no reason to suppose that the time will change 
appreciably any more than the length of our day will. 
The close approach to one of our days, the excess being 
only thirty-seven minutes, leads to the result that, on 
successive nights, Mars will, at the same hour, present 
nearly the same face to the earth. But, owing to the ex- 
cess in question, it will always be a little farther behind 
on any one night than on the night before, so that, at the 
end of forty days, we shall have seen every part of the 
planet that is presented to the earth. 

All that was known of Mars up to a quite recent 
period could be embodied in a map of the planet, showing 
the bright and dark regions of its surface, and in the 
fact that a white cap would be generally seen to surround 
each of its poles. When a pole was inclined toward us, 
and therefore toward the sun, this cap gradually grew 
smaller, enlarging again when the pole was turned from 



180 PLANETS AND THEIR SATELLITES 

the sun. In the latter case it would be invisible from the 
earth, so that the groAvth would be recognised only by 
its larger size when it again came into sight. These caps 
were naturally supposed to be snow and ice which formed 
around the poles during the Martian winter, and partly 
or wholly melted away during the summer. 

The Canals of Mars 

In 1877 commenced Schiaparelli's celebrated observa- 
tions on the surface of Mars, and his announcement of 
the so-called canals. The latter consisted of streaks 
passing from point to point on the planet, and slightly 
darker than the general surface. Seldom has more mis- 
apprehension been caused b}^ a mistranslation than in 
the present case. Schiaparelli called these streaks canale, 
an Italian word meaning channels. He called them so 
because it was then supposed that the darker regions 
of the surface were oceans, and the streams connecting 
the oceans were therefore supposed to be water, and so 
were called channels. But the translation "canals" led 
to a widespread notion that these streaks were the works 
of inhabitants, as canals on the earth are the works of 
men. 

Up to the present time there is some disagreement be- 
tween observers and astronomical authorities on the sub- 
ject of these channels. This arises from the fact that 
they are not well-defined features on an otherwise uni- 
form surface. Everywhere on the planet are found 
variations of shade — light and dark patches, so faint 
and ill defined that it is generally difficult to assign exact 



IfT 



a ■? 



.1 



CO 



I 







"2 ft 2 ■§ 



182 PLANETS AND THEIR SATELLITES 

form and outline to them, running into each other by 
insensible gradations. The extreme difficulty of making 
them out at all, and the variety of aspects they present 
under different illuminations and in different states of 
our atmosphere, has resulted in a great variety of in- 
consistent delineations of these objects. At one extreme 
we have the drawings made by the observers at the Lowell 
Observatory at Flagstaff, Ariz. These show the chan- 
nels as fine dark lines, so numerous as to form a network 
covering the greater part of the surface of the planet. 
In Schiaparelli's map they are rather broad faint bands, 
not nearly so well defined as in Lowell's drawings. Low- 
ell's channels are much more numerous than those seen 
by Schiaparelli. We might therefore suppose that all 
marked by the latter could be identified on Lowell's map. 
But such is far from being the case ; there is only a gen- 
eral resemblance between the features seen at the two 
stations. One of the most curious features of Lowell's 
drawings is that the points where the channels cross each 
other are marked by dark round spots like circular 
lakes. No such spots as these are shown on Schiaparelli's 
map. 

One of the best marked features of Mars is a large, 
dark, nearly circular spot, surrounded by white, which 
is called Lacus Solis, or the Lake of the Sun. All ob- 
servers agree on this. They also agree in a considerable 
part as to certain faint streaks or channels extending 
from this lake. But when we go farther we find that 
they do not agree as to the number of these channels, 
nor is there an exact agreement as to the surrounding 



THE CANALS OF MARS 



183 



features. It will be interesting to study two drawings 
of this region made at the Lick Observatory, probably 
under the best possible conditions, by Campbell and 
Hussey, respectively. 

It is not hkely that any observatory is more favoured 
by its atmosphere for observations on this planet than 
the Lick on Mount Hamilton. Its telescope is the largest 
and finest in the world that has ever been especially 




Figs. 33-34. — Drawings of Lacus Solis on Mars, by Messrs. Campbell and 

Hussey, 



directed to Mars, and Barnard is one of the most cautious 
observers. It is therefore very noteworthy that on the 
face of Mars, as presented to Barnard in the Lick tele- 
scope, the features do not quite correspond to the chan- 
nels of Schiaparelli and Lowell. When the air was ex- 
ceptionally steady he could see a vast number of minute 
and very faint markings, which were not visible in the 
smaller telescopes used by the other observers. These 



184 PLANETS AND THEIR SATELLITES 

were so intricate that it was impossible to represent them 
on a drawing. They were not confined to the brighter 
regions of the planet, or the supposed continents, but 
were found to be more numerous on the so-called seas. 
They showed no such regularity that they could be con- 
sidered as channels running from one region to another. 
The eye could indeed trace darker streaks here and there, 
and some of these corresponded to the supposed channels, 
but they were far more irregular than the features on 
Schiaparelli's and Lowell's maps. 

The matter was explained by Cerulli, a careful and in- 
dustrious Italian observer, in a way which seems very 
plausible. He found that after he had been studying 
INIars for two years he was able, by looking at the moon 
through an opera glass, to see, or fancy he saw, Hnes 
and markings upon its surface similar to those of Mars. 
This phenomenon is not to be regarded as a pure illusion 
on the one hand, or an exact representation of objects 
on the other. It grows out of the spontaneous action of 
the eye in shaping slight and irregular combinations of 
light and shade, too minute to be separately made out, 
into regular forms. 

Probable Nature of the Channels 

The probable facts of the case may be summed up as 
follows : 

1. The surface of Mars is extremely variegated by 
regions differing in shade, and having no very distinct 
outlines. 

2. There are numerous dark streaks, generally some- 



THE ATMOSPHERE OF MARS 185 

what indefinite in outline, extending through consider- 
able distances across the planet. 

3. In many cases the dark portions appear as if 
chained together to a greater or less extent, and thus 
give rise to the appearance of long dark channels. 

The appearance on which this third phenomenon, 
which we may regard as identical with that observed by 
Cerulli, is based, may be well illustrated by looking, with 
a magnifying glass, at a stippled portrait engraved on 
steel. Nothing will then be seen but dots, arranged in 
various lines and curves. But take away the magnifying 
glass and the eye connects these dots into a well-defined 
collection of features representing the outlines of the 
human face. As the eye makes an assemblage of dots into 
a face, so may it make the minute markings on the planet 
Mars into the form of long, unbroken channels. 

The features w^hich we have hitherto described do not 
belong to the two polar regions of the planet. Even when 
the snowcaps have melted away, these regions are seen 
so obliquely that it would be difficult to trace any well- 
defined features upon them. The interesting question is 
whether the caps which cover them are really snow which 
falls during the Martian winter and melts again when 
the sun once more shines on the polar regions. To throw 
light on this question we have to consider some recent 
results as to the atmosphere of the planet. 

The Atmosphere of Mars 

All recent observers are agreed that, if Mars has any 
atmosphere at all, it is much rarer than our own, and 



186 PLANETS AND THEIR SATELLITES 

contains little or no aqueous vapour. This conclusion is 
reached from observations both with the telescope and 
the spectroscope. The most careful eye observations of 
the planet show that the features are rarely, if ever, ob- 
scured by anytliing which can be considered as clouds in 
the ^lartian atmosphere. It is true that the features are 
not always seen with the same distinctness ; but the varia- 
tions in the appearance are no greater than would be due 
to the changes in the steadiness and purity of our own 
atmosphere, through which the astronomer necessarily 
makes his observations. Although, near the edge of the 
apparent disk of the planet, the features appear to be 
softeneli, as if seen through a greater thickness of the at- 
mosphere, this appearance is, at least in part, due to the 
obliquity of the line of sight, which prevents our getting 
so good a view of the edge of the disk as of its centre. 
Something of the same sort may be noticed when the 
moon is viewed with the naked eye or an opera glass. Yet 
it is quite possible that a certain amount of the softening 
may be due to a rare atmosphere on Mars. 

The most careful spectroscopic examination of the 
planet was made by Campbell, who compared its spec- 
trum with that of the moon. He could not detect the 
slightest difference between the two spectra. Now, if 
Mars had an atmosphere capable of exerting a strong 
selective absorption on light, we should see lines in the 
spectrum due to this absorption or, at least, some of the 
lines would be strengthened. Our general conclusion 
therefore must be that, while it is quite probable that 
Mars has an atmosphere, it is one of considerable rarity, 



IS THERE SNOW ON MARS? 187 

and does not bear much aqueous vapour. Now snow can 
fall only through the condensation of aqueous vapour in 
the atmosphere. It does not therefore seem likely that 
much snow can fall on the polar regions of Mars. 

Another consideration is that the power of the sun's 
rays to melt snow is necessarily limited by the amount of 
heat that they convey. In the polar regions of Mars the 
rays fall with a great obliquity, and even if all the heat 
conveyed by them were absorbed, only a few feet of snow 
could be melted in the course of the summer. But 
far the larger proportion of this heat must be reflected 
from the white snow, which is also kept cool by the 
intense radiation into perfectly cold space. We there- 
fore conclude that the amount of snow that can fall 
and melt around the polar regions of Mars must be 
very small, being probably measured by inches at the 
outside. 

As the thinnest fall of snow would suffice to produce a 
white surface, this does not prove that the caps are not 
snow. But it seems more likely that the appearance is 
produced by the simple condensation of aqueous vapour 
upon the intensely cold surface, producing an appear- 
ance similar to that of hoarfrost, which is only frozen 
dew. This seems to me the most plausible explanation 
of the polar caps. It has also been suggested that the 
caps may be due to the condensation of carbonic acid. 
We can only say of this, that the theory, while not impos- 
sible, seems to lack probability. 

The reader will excuse me from saying anything in 
this chapter about the possible inhabitants of Mars. He 



188 PLANETS AND THEIR SATELLITES 

knows just as much of the subject as I do, and that is 
nothing at alL 

The Satellites of Mars 

No discovery more surprised the whole world than that 
of two satellites of Mars by Professor Asaph Hall, at 
the Naval Observatory, in 1877. They had failed of 
previous detection owing to their extreme minuteness. It 
was not considered likely that a satellite could be so small 
as these were found to be, and so no one had taken the 
trouble to make a careful search with any great telescope. 
But, when once discovered, they were found to be by no 
means difficult objects. Of course the ease with which 
they can be seen depends on the position of Mars both in 
its orbit and with respect to the earth. They are never 
visible except when the planet is near its opposition. At 
each opposition they may be observed for a period of 
three, four, or even six months, according to circum- 
stances. At an opposition near periheHon they may be 
seen with a telescope of less than twelve inches diameter ; 
how small a one will show them depends on the skill of the 
observer, and the pains he takes to cut off the light of 
the planet from his eye. Generally a telescope ranging 
from twelve to eighteen inches in diameter is necessary. 
The difficulty in seeing them arises entirely from the 
glare of the planet. Could this be eliminated they could 
doubtless be seen with much smaller instruments. Owing 
to the glare, the outer one is much easier to see than the 
inner one, although the inner one is probably the brighter 
of the two. 



THE SATELLITES OF MARS 189 

Professor Hall assigned the name Deimos to the outer 
and Phobos to the inner, these being the attendants of 
Mars in ancient mythology. Phobos has the remark- 
able peculiarity that it revolves around the planet in 
less than nine hours, making its period the shortest of 
any yet known in the solar system. This is little more 
than one third the time of the planet's rotation on its 
axis. The consequence of this is that, to the inhabitants 
of the planet, its nearest moon rises in the west and 
sets in the east. 

Deimos performs its revolution in 30 hours 18 minutes. 
The result of this rapid motion is that some two days 
must elapse between its rising and setting. 

Phobos is only 3,700 miles from the surface of the 
planet. It must therefore be an interesting object to the 
inhabitants of Mars, if they have telescopes. 

In size these bodies are the smallest visible to us in the 
solar system, with the possible exception of Eros and 
possibly some others of the fainter asteroids. From Pro- 
fessor Pickering's photometric estimates their diameter 
was estim.ated to be not very different from seven miles. 
Their apparent size as we view them is therefore not very 
different from that of a small apple hanging over the 
city of Boston, and seen with a telescope from the city 
of New York. In this respect they form a singular con- 
trast to nearly or quite all of the other satellites, which 
are generall}^ a thousand miles or more in diameter. The 
one exception to this is the fifth satellite of Jupiter, to be 
described in the chapter on Jupiter and its satellites. 
Although this is much less than a thousand miles in diam- 



190 PLANETS AND THEIR SATELLITES 

eter, it must considerably exceed the satellites of Mai*s 
in size. 

The satellites have been most useful to the astronomer 
in enabling him to learn the exact mass of Mars. How 
this is done will be explained in a subsequent chapter, 
where the methods of weighing the planets are set forth. 

The satellites also offer many curious and difficult 
problems in gravitation. Their orbits seem to have a 
slight eccentricity, and the position of the planes in which 
they revolve changes in consequence of the bulging of 
the planet at its equator, produced by its rotation. The 
calculation of these changes and their comparison with 
observations have opened up a field of research in which 
Professor Hermann Struve, now of the University of 
Koenigsberg, Germany, has taken a leading part. 



V 

The Group of Minoe Planets 

The seeming gap in the solar system between the 
orbits of Mars and Jupiter naturally attracted the at- 
tention of astronomers as soon as the distances of the 
planets had been accurately laid down. It became very 
striking when Bode announced his law. There was a 
row of eight numbers in regular progression, and every 
number but one represented the distance of a planet. 
That one place was vacant. Was the vacancy real, or 
was it only because the planet which filled it was so small 
that it had escaped notice? 

This question was settled by Piazzi, an Italian as- 
tronomer who had a little observatory In Palermo in 
Sicily. He was an ardent observer of the heavens, and 
was engaged in making a catalogue of all the stars 
whose positions he could lay down with his instrument. 
On January 1, 1801, he inaugurated the new century 
by finding a star where none had existed before; and 
this star soon proved to be the long-looked-for planet. It 
received the name of Ceres, the goddess of the wheat 
field. 

It was a matter of surprise that the planet should be 
so small; and when its orbit became known it proved to 
be very eccentric. But new revelations were soon to 
come. Before the new planet had completed a revolu- 



192 PLANETS AND THEIR SATELLITES 

tion after its discovery, Dr. Olbers, a physician of Bre- 
men, who employed his leisure in astronomical observa- 
tions and researches, found another planet revolving in 
the same region. Instead of one large planet there were 
two small ones. He suggested that these might be 
fragments of a shattered planet, and that, if so, more 
would probably be found. The latter part of the con- 
jecture proved true. Within the next three years two 
more of these little bodies were discovered, making four 
in all. 

Thus the matter remained for some forty years. 
Then, in 1845, Hencke, a German observer, found a 
fifth planet. The year following a sixth was added, 
and then commenced the curious series of discoveries 
which, proceeding year by year, are now carrying the 
number known rapidly past five hundred. 

Hunting Asteroids 

Up to 1890 these bodies had been found by a few 
observers who devoted especial attention to the search, 
and caught the tiny stars as the hunter does game. They 
would lay traps, so to speak, by mapping the many small 
stars in some small region of the sky near the ecliptic, 
familiarise themselves with their arrangement, and then 
watch for an intruder. Whenever one appeared, it was 
found to be one of the group of minor planets, and the 
hunter put it into his bag. 

Quite a succession of planet-hunters appeared, some 
of them little known for any other astronomical work. 
The most successful of these in the fifties was Gold- 



HUNTING ASTEROIDS 193 

Schmidt, of Paris, a jeweller if I mistake not. Three 
were discovered by Professor James Ferguson at the 
Washington Observatory. Palisa, of Vienna, made a 
record for himself in this work. In this country Pro- 
fessors C. H. F. Peters, of Clinton, and James C. Wat- 
son, of Ann Arbor, were very successful. The last three 
observers carried the number above the two hundred 
mark. 

About 1890 the photographic art was found to offer 
a much easier and more effective means of finding these 
objects. The astronomer would point his telescope at 
the sky and photograph the stars with a pretty long 
exposure, perhaps half an hour, more or less. The stars 
proper would be taken on the negative as small round 
dots. But if a planet happened to be among them it 
would be in motion, and thus its picture would be taken 
as a short Hne, and not as a dot. Instead of scanning 
the heavens the observer had only to scan his photo- 
graphic plate, a much easier task, because the planet 
could be recognised at once by its trail. 

Recently a dozen or more of these bodies have been 
found nearly every year. Of course the unknown ones 
are smaller and more difficult to find as the years elapse. 
But there is as yet no sign of a limit to the number. 
Most of those newly discovered are very minute, yet the 
number seems to increase with their smallness. Even 
the larger of these bodies are so small that they appear 
only as star-like points in ordinary telescopes, and 
their disks are hard to make out even with the most 
powerful instruments. So far as can be determined, 



194 PLANETS AND THEIR SATELLITES 

the diameters of the largest ones, naturally the earliest 
discovered, are only three or four hundred miles. The 
size of the smallest can be inferred only in a rough 
way from their brightness. They may be twenty or 
thirty miles in diameter. 

Orbits of the Asteroids 

The orbits of these bodies are for the most part very 
eccentrico In the case of Polyhymnia, the eccentricity 
is about 0.33, which means that at perihelion it is one 
third nearer the sun than its mean distance, and at aphe- 
lion one third more. It happens that its mean distance 
is just about three astronomical units; its least distance 
from the sun is therefore two, its greatest four, or twice 
as great as the least. 

The large inclination of most of the orbits is also 
noteworthy. In scA^eral cases it exceeds twenty degrees, 
in that of Pallas it is twenty-eight degrees. 

Olbers' idea that these bodies might be fragments of 
a planet which had been shattered by some explosion is 
now abandoned. The orbits range through too wide a 
space ever to have joined, as they would have done if 
the asteroids had once formed a single body. In the 
philosophy of our time these bodies have been as we see 
them since the beginning. On the theory of the nebular 
hypothesis the matter of all the planets once formed 
rings of nebulous substance moving round the sun. In 
the case of all the other planets the material of these 
rings gradually gathered around the densest point of 
the ring, thus agglomerating into a single body. But 



GROUPING OF THE ASTEROIDS 195 

it is supposed that the ring forming the minor plan- 
ets did not collect in this way, but separated into in- 
numerable fragments. 



Groupings of the Orbits 

There is a curious feature of the orbits of these bodies 
which may throw some light on the question of their 
origin. I have explained that the planetary orbits are 
nearly exact circles, but that these circles are not cen- 
tred on the sun. Now imagine ourselves to look down 
upon the solar system from an immense height, and sup- 
pose that the orbits of the minor planets were visible 
as finely drawn circles. These circles would appear to 
interlace and cross 
each other like an 
intricate network, 
filling a broad ring 
of which the outer 
diameter would be 
nearly or quite 
double the inner 
one. 

But suppose we 
could pick all these 
circles up, as if they 
were made of wire, 

and centre them all on the sun, without changing their 
size. The diameters of the larger ones would be double 
those of the smaller, so that the circles would fill a broad 
space, as shown in the figure. Now, the curious fact is 




Fig. 35.- 



-Separation of the Minor Planets 
into Groups. 



196 PLANETS AND THEIR SATELLITES 



that thej would not fill the whole space uniformly, but 
would be collected into distinct groups. These groups 
are shown on the figures of their orbits, given above, and, 
on a different plan, and more com- 
pletely, in the second figure, which is 
arranged on a plan explained as fol- 
lows : Every planet performs its revo- 
lution in a certain number of days, 
which is greater the farther the planet 
from the sun. Since the complete cir- 
cumference of the orbit measures 
1,296,000", it follows that if we 
divide this number by the time of revo- 
lution, the quotient will show through 
what angle, on the average, the planet 
moves along its orbit in one day. This 
angle is called the mean motion of the 
planet. In the case of the minor 
planets it ranges from 400' to more 
than 1,000", being greater the shorter 
the time of revolution and the nearer 
the planet is to the sun. 

Now we draw a vertical line and 
mark off on it values of the mean mo- 
tion, from four hundred to one thou- 
sand seconds, differing by ten sec- 
onds. Between each pair of marks 
we make as many points as there are planets having 
mean motions between the limits. For example, be- 
tween 550" and 560" there are three dots. This means 



750 



■fjUPITER 



Fig. Z&.—Bistribu. 
Hon of the Or- 
bits of the jrinor 
Playlets. 



GROUPING OF THE ASTEROIDS 197 

tliat there are tliree planets having mean motions between 
550" and 560". There are also four planets between 
560" and 570", and one between 570" and 580". Then 
there are no more till Ave pass 610' , when we find six 
planets between 610" and 620", followed by a multitude 
of others. 

Examining the diagram we are able to distinguish 
five or six groups. The outermost one is between 400" 
and 460", and is nearest to Jupiter. The times of revo- 
lution are not far from eight years. Then there is a wide 
gap extending to 560", when we have a group of ten 
planets between 540" and 580". From this point down- 
wards the planets are more numerous, but we find very 
sparse or empt}^ points at 700' , 750 , and 900 . Now 
the most singular feature of the case is that these empty 
spaces are those in which the motion of a planet would 
have a simple relation to that of Jupiter. A planet with 
a mean motion of 900 " would make its circuit round the 
sun in one third the time that Jupiter does ; one of 600" 
in half the time; one of 750" in two fifths of the time. 
It is a law of celestial mechanics that the orbits of planets 
having these simple relations to another undergo great 
changes in the course of time from their action on each 
other. It was therefore supposed by Kirkwood, who first 
pointed out these gaps in the series, that they arose be- 
cause a planet within them could not keep its orbit per- 
manently. But it is curious that there is no gap, but on 
the contrary, a group of planets whose mean motion is 
nearly two thirds of that of Jupiter. Hence the view is 
doubtful. 



198 PLANETS AND THEIR SATELLITES 

The Most Curious of the Asteroids 

One of these bodies is so exceptional as to attract our 
special attention. All the hundreds of minor planets 
known up to 1898 moved between the orbits of Mars 
and Jupiter. But in the summer of that year Witt, of 
Berlin, found a planet which, at perihelion, came far 
within the orbit of IMar^ — in fact within fourteen million 
miles of the orbit of the earth. He named it Eros. 
The eccentricity of its orbit is so great that at aphelion 
the planet is considerably outside the orbit of Mars. 
Moreover the two orbits, that of the planet and of Mars, 
pass through each other like two links of a chain, so 
that if the orbits were represented of wire they would 
hang together. 

Owing to the inclination of its orbit, this planet 
seems to wander far outside the limits of the zodiac. 
When nearest the earth, as it was in 1900, it was for a 
time so far north that it never set in our middle lati- 
tudes, and passed the meridian north of the zenith. This 
peculiarity of its motion was doubtless one reason why 
it was not found sooner. During its near approach in 
the winter of 1900-'01 it was closely scrutinised and 
found to vary in brightness from hour to hour. Care- 
ful observation showed that these changes went through 
a regular period of about two and a half hours. At 
this interval it would fade away a little with great uni- 
formity. Some observers maintained that it was fainter 
at every alternate minimum of light, so that the real 
period was five hours. It was supposed that this indi- 



MOST CURIOUS OF THE ASTEROIDS 199 

cated that the object was really made up of two bodies 
revolving round each other — perhaps actually joined 
into one. But it seems more likely that the variations 
of light were due to there being light and dark regions 
on the surface of the little planet, which therefore 
changed in brightness according as bright or dark 
regions predominated on the surface of the hemisphere 
turned toward us. The case was made perplexing by 
the gradual disappearance of the variations after they 
had been well established by months of observation. 
There seems to be some mystery in the constitution of 
this bodj^ 

From a scientific point of view Eros Is most interesting 
because, coming so near the earth from time to time, 
its distance may be measured with great precision, and 
the distance of the sun as well as the dimensions of the 
whole solar system thus fixed with greater exactness 
than by any other method. Unfortunately, the nearest 
approaches occur only at very long intervals. What 
is most tantalising is that there was such an approach in 
1892 before the object was recognised. At that time it 
was photographed a number of times at the Harvard 
Observatory, but was lost in the mass of stars by which 
it was surrounded. Its distance was, astronomically, only 
sixteen hundredths, or some fifteen millions of miles, 
while the nearest approaches of Mars are nearly forty 
millions. There will not be another approach so near for 
more than sixty, perhaps not for more than a hundred 
years. 

In 1900 it approached the earth within about thirty 



200 PLANETS AND THEIR SATELLITES 

millions of miles, and a combined effort was made at 
various observatories to lay down its exact position from 
night to night among the stars by photography, with a 
view to determining its parallax. But the planet was 
faint, the observations were difficult, and it is not yet 
known what measure of success was reached. 

Variations of light which might be due to a rotation 
on their axes have been suspected in the case of other 
asteroids besides Eros, but nothing has yet been settled. 



VI 

Jupiter and its Satellites 

Jupiter, the "giant planet," is, next the sun, the 
largest body of the solar system. It is, in fact, more than 
three times as large, and about three times as massive as 
all the other planets put together. Yet, such is the pre- 
ponderating mass of our central luminary that the mass 
of Jupiter is less than one thousandth part that of the 
sun. 

This planet is in opposition in September, 1903, Octo- 
ber, 1904, November, 1905, and so on for several years 
afterward, about a month later every year. Near the 
time of opposition it may easily be recognised in the even- 
ing sky, both by its brightness and its colour. It is then, 
next to Venus, the brightest star-like object in the 
heavens. It can easily be distinguished from Mars by its 
whiter colour. If we look at it with a telescope of the 
smallest size, even with a good ordinary spy-glass, we 
shall readily see that instead of being a bright point, like 
a star, it is a globe of very appreciable dimensions. We 
shall also see what look like two shadowy belts crossing 
the disk. These were noticed and pictured two hundred 
years ago by Huygens. As greater telescopic power was 
used it was found that these seeming belts resolved them- 
selves into very variegated cloud-like forms, and that 
they vary, not only from month to month, but even from 



202 PLANETS AND THEIR SATELLITES 

night to night. By careful observation on the aspect 
which thej present from hour to hour, and from night to 
night, it was found that the planet rotates on its axis 
in about 9 hours 55 minutes. The astronomer may there- 
fore in the course of a single night see every part of the 
surface of the planet presented to his view in succession. 

Two features presented by the planet will at once 
strike the careful observer with the telescope. One of 
these is that the disk does not seem uniformly bright, but 
gradually shades off near the limb. The latter, instead 
of being bright and hard is somewhat soft and diffuse. 
In this respect the appearance forms quite a contrast to 
that presented by the moon or Mars. The shading off 
toward the edge Is sometimes attributed to a dense atmos- 
phere surrounding the planet. While this is possible, 
it is by no means certain. 

The other feature to which we allude is an ellipticity of 
the disk. Instead of being perfectly round, the planet 
is flattened at the poles, like our earth, but in a much 
greater degree. The most careful observer, viewing the 
earth from another planet, would see no deviation from 
the spherical form, but, viewing Jupiter, the deviation is 
very perceptible. This is owing to its rapid rotation on 
its axis, which causes its equatorial regions to bulge out, 
as, to a smaller degree, in the case of the earth. 

Surface of Jupiter 

The features of Jupiter, as we see them with a tele- 
scope, are almost as varied as those of the clouds which 
we see in our atmosphere. There are commonly elon- 



SURFACE OF JUPITER W3 

gated strata of clouds, apparently due to the same cause 
that produces stratified clouds on the earth, namely, cur- 
rents of air. Among these clouds round white spots are 
frequently seen. The clouds are sometimes of a rosy 
tinge, especially those near the equator. They are 
darkest and most strongly marked in middle latitudes, 
both north and south of the equatorial regions. It is 
this that produces the appearance of dark belts in a small 
telescope. 

The appearance of Jupiter is, in almost every point, 
very different from that of Mars or Venus. Comparing 
it with Mars, the most strongly marked difference con- 
sists in the entire absence of permanent features. Maps 
of Mars ma}^ be constructed and their correctness tested 
by observations generation after generation, but owing 
to the absence of permanence, no such thing as a map of 
Jupiter is possible. 

Notwithstanding this lack of permanence, features 
have been know^n to endure through a number of years, 
and some of them may be permanent. The most remark- 
able of these was the great red spot, which appeared in 
middle latitudes, on the southern hemisphere of the planet, 
about the year 1878. For several years it was a very 
distinct object, readily distinguished by its colour. After 
ten years it began to fade awa}^, but not at a uniform 
rate. Sometimes it would seem to disappear entirely, 
then would brighten up once more. These changes con- 
tinued but, since 1892, faintness or invisibility has been 
the rule. If the spot finally disappeared, it was in so un- 
certain a way that no exact date for the last observation 




« 

^ 



Si 



•s 



CONSTITUTION OF JUPITER W5 

of it can be given. Some observers with good eyes still 
report it to be visible from time to time. 

Constitution of Jupiter 

The question of the constitution of this curious planet 
is still an unsettled one. There is no one hypothesis that 
readily explains all the facts, which suggest many points, 
but prove few, unless negativel}^ 

Perhaps the most remarkable feature of the planet is 
its small density. Its diameter is about eleven times that 
of the earth. It follows that, in volume, it must exceed 
the earth more than thirteen hundred times. But its 
mass is only a little more than three hundred times that 
of the earth. It follows from this that its density is much 
less than that of the earth ; as a matter of fact, it is only 
about one third greater than the density of water. A 
simple computation shows that the force of gravity at its 
surface is between two and three times that at the surface 
of the earth. Under this enormous gravitation we might 
suppose its interior to be enormously compressed, and its 
density to be great in comparison. Such would certainly 
be the case were it made up of solid or fluid matter of the 
same kind that composes the surface of the earth. From 
this fact alone the conclusion would be that its outer por- 
tions at least were composed of aeriform matter. But 
how reconcile this form with the endurance of the red 
spot through twenty-five years? This is the real diffi- 
culty of the case. 

Nevertheless, the hypothesis is one which we are forced 
to accept without great modification. Besides the evi- 



S06 PLANETS AND THEIR SATELLITES 

deuce of vapour as shown b}^ the constantly changing 
aspect of the planet, we have another almost conclusive 
piece of evidence in the law of rotation. It Is found that 
Jupiter resembles the sun in that its equatorial region 
rotates in less time than the regions north of middle lati- 
tude, although the circuit they have to make is longer. 
This is probably a law of rotation of gaseous bodies in 
general. It seems, therefore, that Jupiter has a greater 
or less resemblance to the sun in its physical constitution, 
a view which quite corresponds with Its aspect In the tele- 
scope. The difference in the time of rotation at the equa- 
tor and in middle latitudes is, so far as we yet know, about 
five minutes. That Is to say, the equatorial region rotates 
in nine hours fifty minutes and those In middle latitudes in 
nine hours fifty-five minutes. This corresponds to a dif- 
ference of velocity of the motion between the two amount- 
ing to about two hundred miles an hour; a seemingly 
impossible difference were the surface liquid. 

It is a singular fact that no well-defined law of rotation 
in different latitudes has 3"et been made out, as has been 
done In the case of the sun. Were we to accept the re- 
cults of the meagre observations at our disposal we might 
be led to the conclusion that the difference of time is 
not a graduall}^ varying quantltj^, as we go from the 
equator toward the poles, but that the change of five 
minutes occurs very near a certain latitude and almost 
suddenly. But we cannot assume this to be the case 
without more observations than are yet on record. The 
subject Is one of which an accurate investigation is 
greatly to be desired. 



CONSTITUTION OF JUPITER 207 

Yet another resemblance between Jupiter and the sun 
is that the J are both brighter in the centre of their disk 
than toward the circumference. In the case of Jupiter, 
the shading off is very well marked. The extreme cir- 
cumference especially is more softened than that of any 
of the other planets. 

The apparent resemblance between the surfaces of 
these bodies, taken in connection with the brightness of 
the planet, has led to the question whether Jupiter may 
not be, in whole or in part, self-luminous. This again is 
a question which needs investigation. The idea that the 
planet can emit much light of its own seems to be nega- 
tived by the fact that the satellites completely disappear 
when they pass into its shadow. We may therefore say 
with entire certainty that Jupiter does not give enough 
light to enable us to see a satellite by that light alone. 
We can hardly suppose that this would be the case if the 
satellite received one per cent as much light from the 
planet as it does from the sun. It is also found that the 
light which Jupiter sends out is somewhat less than 
that which it receives from the sun. That is to say, all 
the light which It gives out, when estimated in quantity, 
may be reflected light, without supposing the planet 
brighter than white bodies on the surface of the earth. 
But this still leaves open the question whether the white 
spots, sometimes so much brighter than the rest of the 
planet, may not give us more light than can fall upon 
them. This also is a question not yet investigated. 

The hypothesis which best lends itself to all the facts 
seems to be that the planet has a solid nucleus, of which 



208 PLANETS AND THEIR SATELLITES 

the density may be as great as that of the earth or any 
other sohd planet, and that the small average density 
of the entire mass is due to the vapourous character of the 
matter which surrounds this nucleus. In all probability 
the nucleus is . at a very high temperature, even ap- 
proximating that at the surface of the sun, but this 
temperature gradually diminishes as we ascend through 
the gaseous atmosphere, as we suppose to be the case 
with the sun; hence it may happen that, at the surface, 
none of the material that we see is at a high enough 
temperature to radiate a sensible amount of heat. 

On the whole we may describe Jupiter as a small sun 
of which the surface has cooled off till it no longer emits 
light. 

The Satellites of Jupiter 

When Galileo first turned his little telescope on the 
planet Jupiter he was delighted and surprised to find it 
accompanied by four minute companions. Watching 
them from night to night, he found them to be in rev- 
olution around their central body as, upon the theory 
not fully accepted in his time, the planets revolve 
around the sun. This remarkable resemblance to the 
solar system was a strong point in favor of the Coper- 
nican Theory. 

These bodies can be seen with a common spy-glass, or 
even a good opera glass. It has even been supposed that 
good eyes sometimes see them without optical assistance. 
They are certainly as bright as the smallest stars visible 
to the naked eye, y^t the glare of the planet would seem 
to be an insuperable obstacle to their visibility, even to 



THE SATELLITES OF JUPITER 209 

the keenest vision. A story has been told, by Arago, I 
think, of a woman who professed to be able to see them at 
any time and even pointed out their positions. It was 
found, however, that she described them as on the op- 
posite side of the planet to that on which they were really 
situated. It was then found, or inferred, that she took 
the positions from an astronomical ephemeris, in which 
diagrams of them were given, but in which the pictures 
were made upside down in order that the satellites might 
be seen as in an ordinary inverting telescope. But it 
seems quite likely that, when the two outer satellites 
chance to be nearly in the same straight line, they may 
be visible by their combined light. 

From the measures of Barnard it may be inferred that 
these bodies range somewhere between two and three thou- 
sand miles in diameter. Hence, they do not differ greatly 
from our moon in size. 

Only four satellites were known until 1892 ; then Bar- 
nard, with the great Lick telescope, discovered a fifth, 
much nearer the planet than the four others. It makes 
a revolution in a little less than twelve hours, the short- 
est periodic time known except that of the inner satellite 
of Mars. Still, however, it is a little longer than the 
rotation time of the planet. The next outer one, or the 
innermost of the four previously known, still called the 
first satellite, revolves in about one day eighteen and a 
half hours, while the outer one requires nearly seventy 
days to perform its circuit. 

In its visibility the fifth satellite is the most difficult 
known object in the solar system. Through only a few 



SIO PLANETS AND THEIR SATELLITES 

of the most powerful telescopes of the world has it ever 
certainly been seen by the human eye. Its orbit is de- 
cidedly eccentric. Owing to the ellipticity of the planet, 
it possesses the remarkable peculiarity that its major axis, 
and, therefore, the perihelion point of its orbit, performs 
a complete rcA^olution in about a year. 

It has sometimes been questioned whether these satel- 
lites are round bodies, like the planets and most other 
satellites. Some observers, especially Barnard and W. H. 
Pickering, noticed curious changes in the form of the 
first satellite as it was crossii^g the surface of the planet. 
At one time it looked like a double body. But Barnard, 
by careful and repeated study, showed that this appear- 
ance was partly due to the varying shade of the back- 
ground on which the satellite was seen projected upon 
the planet, and partly to the differences in the shade of 
various parts of the satellite itself. 

During their course around the planet these bodies 
present many interesting phenomena, which can be ob- 
served with a moderate sized telescope. These are their 
eclipses and transits. Of course Jupiter, like any other 
opaque body, casts a shadow. As the satellites make 
their round they nearly always pass through the shadow 
during that part of their course which is beyond the 
planet. Exceptions sometimes occur in the case of the 
fourth and most distant satellite, which may pass above 
or below the shadow, as our moon passes above or below 
that of the earth. When a satellite enters the shadow, it 
is seen to fade away gradually, and finally to disappear 
from sight altogether. 



THE SATELLITES OF JUPITER 211 

For the same reason the satellites generally pass across 
the disk of the planet in that part of their course which 
lies on this side of it. The general rule is that, when a 
satellite has impinged on the planet, it looks brighter 
than the latter, owing to the darkness of the planet's limb. 
But, as it approaches the central regions, it may look 
darker than the background of the planet. Of course 
this does not arise from any change in the brightness of 
the satellite, but only from the fact, already mentioned, 
that the planet is brighter in its central regions than at 
its limb. 

Yet more interesting and beautiful is the shadow of a 
satellite which, under such circumstances, may often be 
seen upon the planet, looking like a black body crossing 
alongside the satellite itself. Such a shadow is shown in 
the picture of Jupiter on page 204. 

The phenomena of Jupiter's satellites, including their 
transits and those of their shadows, are all predicted in 
the astronomical ephemerides, so that an observer can 
always know when to look for an eclipse or transit. 

The eclipses of the inner of the four older satellites 
occur at intervals of less than two days. By noting their 
times, an observer in unknown regions of the earth can 
determine his longitude more easily than by any other 
method. He has first to determine the error of his watch 
on local time by certain simple astronomical observations, 
quite familiar to astronomers and navigators. He thus 
finds the local time at which an eclipse of the satellite 
takes place. He compares this with the time predicted 
in the ephemeris. The difference gives his longitude 



212 PLANETS AND THEIR SATELLITES 

according to the S3^stem set forth in our chapter on Time 
and Longitude. 

The principal drawback of this method is that it is 
not very accurate. Observations of the time of such an 
echpse are doubtful to a large fraction of a minute. This 
corresponds to 15 minutes of longitude, or 15 nautical 
miles at the equator. In the polar regions the effect of 
the error is much smaller, owing to the convergence of 
the meridians. The method is, therefore, most valuable 
to polar explorers. 



VII 

Saturn and its System 

Among the planets, Saturn is next to Jupiter in size 
and mass. It performs its revolution round the sun in 
twenty-nine and a half years. When the planet is visible 
the casual observer will generally be able to recognise 
it without difficulty by the slightly reddish tint of its 
light, and by its position in the heavens. During the 
next few years it will be in opposition first in summer 
and then in autumn, about twelve or thirteen days later 
each year. Starting from August, 1903, opposition will 
occur in August of 1904-'05, September of 1906-'08, Oc- 
tober of 1909-'! 0, and so on. At these times Saturn will 
be seen each evening after dark in the eastern or south- 
eastern sky, moving toward the south as the evening ad- 
vances. It looks a good deal like Arcturus, which, for a 
few years to come, will be visible at the same seasons, 
only high up in the south or southwest, or lower down in 
the west. 

Although Saturn is far from being as bright as Jupi- 
ter, its rings make it the most magnificent object in the 
solar sj^stem. There is nothing else like them in the 
heavens, and it is not surprising that they were an 
enigma to the early observers with the telescope. To 
Galileo they first appeared as two handles to the planet. 
After a year or two they disappeared from his view. We 



214 PLANETS AND THEIR SATELLITES 

now know that this occurred because, owing to the motion 
of the planet in its orbit, they were seen edge-on, and 
are then so thin as to be invisible in a telescope as imper- 
fect as Galileo's. But the disappearance was a source 
of great embarrassment to the Tuscan philosopher, who 
is said to have feared that he had been the victim of some 
illusion on the subject, and ceased to observe Saturn. 
He was then growing old, and left to others the task of 
continuing his observations. Of course the handles soon 
reappeared, but there was no way of learning what they 
were. After more than forty years the riddle was solved 
by Huyghens, the great Dutch astronomer and physicist, 
who announced that the planet was surrounded by a thin 
plane ring, nowhere touching it, and inclined to the 
ecliptic. 

Satellites of Saturn 

Besides his rings, Saturn is surrounded by a retinue of 
eight satellites — a greater number than any other planet. 
The existence of a ninth has been suspected, but awaits 
confirmation. They are very unequal in size and dis- 
tance from the planet. One, Titan, may be seen with a 
small telescope ; the faintest, only in very powerful ones. 

Titan was discovered by Huyghens just as he had 
made out the true nature of the rings. And hereby 
hangs a little tale which has only recently come out 
through the publication of Huyghens's correspondence. 
Following a practice of the time, the astronomer sought 
to secure priority for his discovery without making it 
known, by concealing it in an anagram, a collection of 
letters which, when properly arranged, would inform the 



ASPECTS OF SATURN'S RINGS 215 

reader that the companion of Saturn made its revolu- 
tion in fifteen days. A copy of this was sent to WalHs, 
the celebrated English mathematician. In his reply the 
latter thanked Huyghens for his attention, and said he 
also had something to say, and gave a collection of letters 
longer than that of Huyghens. When the latter inter- 
preted his anagram to Wallis, he was surprised to receive 
in reply a solution of the Wallis anagram announcing 
the very same discovery, but, of course, in different lan- 
guage and at greater length. It turned out that Wallis, 
who was expert in ciphers, wanted to demonstrate the 
futility of the system, and had managed to arrange his 
own letters so as to express the discovery, after he knew 
what it was. Huyghens did not appreciate the joke. 

Varying Aspects of Saturn'' s Rings 

The Paris Observatory was founded in 1666 as one of 
the great scientific institutions of France which adorned 
the reign of Louis XIV. Here Cassini discovered the 
division in the ring, showing that the latter was really 
composed of two, one outside the other, but in the same 
plane. The outer of these rings has somewhat the ap- 
pearance of being again divided, by a line called the 
Encke division, after the astronomer who first noticed it, 
but the exact nature of this division is still in doubt. It 
certainly is not sharp and well defined like the Cassini 
division, but only a slight shade. 

To understand the varying appearance of the rings 
we give a figure showing how they and the planet would 
look if we could see them perpendicularly (which we 



216 PLANETS AND THEIR SATELLITES 



never can). We notice first the dark Cassini division, 
separating the rings into two, an inner and an outer one, 
the latter being the narrower. Then, on the outer ring, 
we see the faint and grey Encke division, which is much 

less marked and much 
harder to see than 
the other. Passing 
to the inner ring, 
the latter shades off 
gradually on the in- 
ner edge, where there 
is a grey border 
called the "crape 
ring." This was first 
described by Bond, 
of the Harvard Ob- 
servatory, and was 
long supposed to be 
a separate and dis- 
tinct ring. But careful observation shows that such is 
not the case. The crape ring joins on to the ring out- 
side of it, and the latter merely fades away into the 
other. 

The rings of Saturn are inclined about twenty-seven 
degrees to the plane of its orbit, and they keep the same 
direction in space as the planet revolves round the sun. 
The effect of this will be seen by the figure, which shows 
the orbit of the planet round the sun in perspective. 
When the planet is at A the sun shines on the north 
(upper) side of the ring. Seven years later, when the 




Fig. 39. — Perpendicular View of the Rings 
of Saturn. 



ASPECTS OF SATURN'S RINGS 217 

planet is at B, the ring is presented to the sun edgewise. 
After passing B the sun shines on the south (lower) side 
at an inclination which continually increases till the 
planet makes C, when the inclination is at its greatest, 

._.....^^ 






C\ 



Fig. 40. — Shoioing how the Direction of the Plane of Saturn''s Hings re- 
mains Unchanged as the Planet moves round the Sun. 

twenty-seven degrees. Then it diminishes as the planet 
passes to D, at which point the edge of the ring is again 
presented to the sun. From this point to A and B the 
sun again shines on the north side. 

The earth is so near the sun in comparison with Saturn 
that the rings appear to us nearly as they would to an 
observer on the sun. There is a period of fifteen years, 
during which we see the north side of the ring, and at the 
middle of which we see them at the widest angle. As 
the years advance, the angle grows narrower and the 
rings are seen more and more edgewise till they close up 
into a mere line crossing the planet, or perhaps disappear 
entirely. Then they open out again, to close up in 
another fifteen years. A disappearance occurred in 1892 
and another will take place in 1907. 



218 PLANETS AND THEIR SATELLITES 

With this view of what the shape of the rings really is, 
we may understand their appearance to us. The rings 
are always seen very obliquely, never at a greater angle 
than twenty-seven degrees. The general outline pre- 




FiGS. 41-42.— 



arance of the Rings of Satiirn, according to Bar- 
nard^ when seen edgeioise. 



sented by the planet and rings is that seen in Figure 40. 
The best views are obtained when the rings are seen at a 
considerable angle. The divisions and the crape ring 
are then seen. The shadow of the globe of the planet on 
the ring will be seen as a dark notch. A dark line cross- 



WHAT THE RINGS ARE 219 

ing the planet like a border to the inner ring is the 
shadow of the ring on the planet. 

Very interesting are rather rare occasions when the 
plane of the ring passes between the earth and the sun. 
Then the sun shines on one side of the ring while the 
other side is presented to us, though, of course, at a very 
small angle. The chances for observing Saturn at such 
times are rather few, especially in recent times. At both 
the last occasions, 1877 and 1892, this only happened 
for a few days, when the planet w^as not well situated for 
these observations. Nevertheless, in October, 1892, Bar- 
nard got a look at it from the Lick Observatory, and 
found that the rings were totally invisible, though their 
shadow could be seen on the planet. This shows that the 
rings are so thin that their edges are invisible in a 
powerful telescope. 

What the Rings are 

When it became accepted that the laws of mechanics, 
as we learn them on the earth, govern the motions of the 
heavenly bodies, another riddle was presented by the 
rings of Saturn. What keeps the rings in place .? What 
keeps the planet from running against the inner ring 
and producing, to modify Addison's verse, a "wreck of 
matter and crush of worlds" that would lay the whole 
beautiful structure in ruins ? It was for a time supposed 
that a liquid ring might be proof against such a catas- 
trophe, and then it was shown that such was not the case. 
Finally it was made clear that the rings could not be co- 
hering bodies of any kind, but were merely clouds of 



220 PLANETS AND THEIR SATELLITES 

minute bodies, perhaps little satellites, perhaps only par- 
ticles like pebbles or dust, or perhaps like a cloud of 
smoke. This view had to be accepted, but was long with- 
out direct proof. The latter was finally brought out by 
Keeler with his spectroscope. He found that when the 
light of the rings was spread out into a spectrum, the 
dark spectral lines did not go straight across it, but were 
bent and broken in such a way as to show that the matter 
of the rings was revolving round the planet at unequal 
speeds. At the outer edge it revolved most slowly; the 
speed continually increased toward the inner edge, and 
was everywhere the same that a satellite would have if it 
revolved round the planet at that distance. This most 
beautiful discovery was made at the Allegheny Observa- 
tory near Pittsburg, Pa. 

System of Saturn's Satellites 

In making known his discovery of the satellite Titan, 
Huyghens congratulated himself that the solar system 
was now complete. There were now seven great bodies 
and seven small ones, the magic number of each. But 
within the next thirty years Cassini exploded all this 
mysticism by discovering four more satellites. Then, 
after the lapse of a century, the great Herschel found 
yet two more. Finally, the eighth was found by Bond 
at the Harvard Observatory in 1848. 

In 1898 photographs of the sky taken at the South 
American branch of the Harvard Observatory showed a 
star near Saturn, but farther than the outermost known 
satellite, which seemed to be in a different position each 



SATELLITES OF SATURN 



221 



night. It has not yet been decided whether this was a 
sateUite, because Saturn has been among the countless 
faint stars of the Milky Way, among which the satellite 
might be lost. 

The following is a list of the eight satellites, with 
their distances from the planet in radii of the latter, 
their times of revolution, and the discoverer of each : 









Date 


Distance 


Time 


No. 


Name. 


Discoverer. 


of Dis- 


from 
Planet. 


of Revo- 








covery. 


lution. 












d. h. 


1 


Mimas 


Herschel 


1789 


3.3 


23 


2 


Enceledas. 


Herschel 


1789 


4.3 


1 9 


3 


Tethys 


Cassini 


1684 


5.3 


1 21 


4 


Dione 


Cassini 


1684 


6.8 


2 18 


5 


Rhea 


Cassini 


1672 


9.5 


4 12 


6 


Titan 


Huyghens 


1655 


21.7 


15 23 


7 


Hyperion. . 
Japetus . . . 


Bond 


1848 


26 8 


21 7 


8 


Cassini 


1671 


64.4 


70 22 



The most noteworthy features of this list are the wide 
range of distances among the satellites, and the relation 
between the times of revolution of the four inner ones. 
The five inner ones seem to form a group by themselves. 
Then there is a gap exceeding in breadth the distance of 
the innermost of the five, when we have another group of 
two. Titan and Hyperion. Then there is a gap wider 
than the distance of Hyperion, outside of which comes 
Japetus, the outermost yet known. 

A curious relation among the periods is that the period 
of the third satellite is almost exactly twice that of the 
first; and that of the fourth almost twice that of the 



222 PLANETS AND THEIR SATELLITES 

second. Also, four periods of Titan are almost exactly 
equal to three of Hyperion. 

The result of the latter relation is a certain very curi- 
ous action of these two satellites on each other, through 
their mutual gravitation. To show this we give a dia- 
gram of the orbits. That of Hyperion, the outer of the 




Fig. 43. — Orbits of Titan and Hyperioii^ showing their relation. 



two, is very eccentric, as will be seen by the figure. Sup- 
pose the satellites to be in conjunction at a certain mo- 
ment ; Titan, the inner and larger of the two at a point 
A, Hyperion at the point a just outside. At the end of 
sixty-five days Titan will have made three revolutions 
and Hyperion four, which will bring them again into 



SATELLITES OF SATURN 223 

conjunction at very nearly, but not exactly, the same 
point. Titan will have reached the point B, and Hy- 
perion b. At a third conjunction the two will be a little 
above the line Bb, and so on. Really the conjunctions 
occur closer together than we have been able to draw 
them in the figure. In the course of nineteen years the 
point of conjunction will have slowly moved all round 
the circle, and the satellites will again be in conjunction 
at A. 

Now the effect of this slow motion of the conjunction- 
point round the circle is that the orbit of Hyperion, or, 
more exactly, its longer axis, is carried round with the 
conjunction-point, so that the conjunctions always occur 
where the distance of the two orbits is greatest. The 
dotted line shows how the orbit of Hyperion is thus car- 
ried halfway round in nine years. 

An interesting feature of this action is that it is, so 
far as we know, unique, there being no case like it else- 
where in the solar system. But there may be something 
quite similar in the mutual action of the first and third, 
and of the second and fourth satellites of Saturn on each 
other. 

A yet more striking effect of the mutual attraction of 
the matter composing the rings and satellites is that, 
excepting the outer satellite of all, these bodies all keep 
exactly in the same plane. The effect of the sun's at- 
traction, if there were nothing to counteract it, would 
be that in a few thousand years the orbits of these bodies 
would be drawn around into different planes, all having, 
however, the same inclination to the plane of the orbit 



224 PLANETS AND THEIR SATELLITES 

of Saturn. But, by their mutual attraction, the planes 
of the orbits are all kept together as if they were solidly 
attached to the planet. 

Physical Constitution of Saturn 

There is a remarkable resemblance between the phys- 
ical make-up of this planet and that of its neighbour 
Jupiter. They are alike remarkable for their small den- 
sity, that of Saturn being even less than that of water. 
Another point of likeness is the rapid rotation, Saturn 
turning on its axis in 10 hours 14 minutes, a little more 
than the period of Jupiter. The surface of the planet 
also seems to be variegated with cloud-like forms, similar 
to those of Jupiter, but far fainter, so that they cannot 
be seen with any distinctness. 

What has been said of the probable cause of the small 
density of Jupiter applies equally to Saturn. The prob- 
ability is that the planet has a comparatively small but 
massive nucleus, surrounded by an immense atmosphere, 
and that what we see is only the outer surface of the 
atmosphere. 

A curious fact which bears on this view is that the 
satellite Titan is far denser than the planet. Its cubical 
contents are about one ten-thousandth those of the planet. 
But its mass, as found from the motion of Hyperion, is 
one forty-three-hundredth that of the planet. 



VIII 

Uranus and its Satellites 

Uranus is the seventh of the major planets in the order 
of distance from the sun. It is commonly considered a 
telescopic planet; but one having good eyesight can 
easily see Uranus without artificial help, if he only knows 
exactly where to look for it, so as to distinguish it from 
the numerous small stars having the same appearance. 
Had any of the ancient astronomers made so thorough an 
examination of the sky from night to night as Dr. Gould 
did of the southern heavens after he founded the Cordoba 
Observatory, they would have upset the notion that there 
were only seven planets. 

Uranus was discovered in 178S by Sir William Her- 
schel, who at first supposed it to be the nucleus of a 
comet. But its motion soon showed that this could not be 
the case, and before long the discoverer found that it 
was a new addition to the solar system. In gratitude to 
his royal benefactor, George III, he proposed to call the 
planet Georgium Sidus, a name which was continued in 
England for some seventy years. Some continental as- 
tronomers proposed that it should be called after its 
discoverer, and the name Herschel was often assigned to 
it. But by 1850 the name Uranus, originally proposed 
by Bode (author of the "Law"), and always used in 
Germany, became universal. 



226 PLANETS AND THEIR SATELLITES 

When the orbit of the planet was determined, so that 
its course in former years could be mapped out, the curi- 
ous fact was brought to Hght that it had been seen and 
recorded nearly a century before, as well as a few years 
previously. Flamsteed, Astronomer Royal of England, 
while engaged in cataloguing the stars, had marked it 
down as a star on five occasions between 1690 and 1715. 
What was yet more singular, Lemonnier, at the Paris Ob- 
servatory, had recorded it eight times in the course of 
two months, December, 1768, and January, 1769. But 
he had never reduced and compared his observations, and 
not till Herschel announced the planet did Lemonnier 
know how great a prize had lain for ten years within 
liis grasp. 

The period of revolution of Uranus is eighty-four 
years, so that its position in the sky changes but slowly 
from year to year. During the first ten years of our cen- 
tury it will be in or near the region of the Milky Way, 
which we see in summer and autumn, low down in the 
southern sky. This will make it difficult of detection by 
the naked eye. 

The distance of Uranus is about twice that of Saturn. 
In astronomical units it is 19.2; in our familiar measures 
1,790,000,000 miles, or 2,870,000,000 kilometres. 

Owing to this great distance, it is hard to see with cer- 
tainty any features on its surface. In a good telescope 
it appears as a pale disk with a greenish hue. Some ob- 
servers have fancied that they saw faintly marked fea- 
tures on its surface, but this is probably an illusion. We 
may regard it as certain that it rotates on its axis ; but 



THE SATELLITES OF URANUS 227 

no ocular evidence of this has ever been obtained, and of 
course the period is unknown. But the measures of Bar- 
nard showed a shght elHpticity of the disk which, if real, 
would prove a rapid rotation. 

The spectroscope shows that the constitution of 
Uranus is materially different from that of any of the 
six planets which revolve between it and the sun. None 
of the latter gives a spectrum which is strikingly different 
from that of ordinary sunlight. But when the light of 
Uranus is spread out into a spectrum, a number of more 
or less shaded bands are seen, totally unlike the lines of 
an ordinary spectrum. Whether these bands are really 
what they appear, or whether they are composed of a 
multitude of fine dark lines invisible singly, owing to 
the faintness of the light, has not yet been ascertained; 
but the probabilities are that such is the case. Whether 
it is or not, the spectrum indicates that the light reflected 
from the planet has passed through a dense medium of a 
constitution quite different from that of our atmosphere. 
But it is as yet impossible to determine the nature of this 
medium. 

The Satellites of Uranus 

There are four of these bodies moving round Uranus 
as he travels in his orbit. The two outer ones can be 
seen in a telescope of twelve inches aperture or more ; the 
inner ones only in the most powerful telescopes of the 
world. The difficulty of seeing them does not arise from 
their small size, for they are probably nearly or quite as 
large as the others, but from their being blotted out by 
the glare of the planet. 



228 PLANETS AND THEIR SATELLITES 

The history of these bodies is somewhat pecuhar. Be- 
sides the two brighter ones, Herschel, before 1800, 
thought he caught ghmpses from time to time of four 
others, and thus it happened that for more than half a 
century Uranus was credited with six satellites. This 
was because during all that time no telescope was made 
which could claim superiority over Herschel's. 

Then about 1845, Lassell, of England, undertook the 
making of reflecting telescopes, and produced his two 
great instruments, one of two, the other of four feet 
aperture. The latter he afterwards took to the Island 
of Malta, in order to make observations under the fine 
sky of the Mediterranean. Here he and his assistant 
entered upon a careful examination of Uranus, and 
reached the conclusion that none of the additional satel- 
lites supposed by Herschel had any existence. But, on 
the other hand, two new ones were found so near the 
planet that they could not have been seen by any pre- 
vious observer. During the next twenty years these 
newly found bodies were looked for in vain with the best 
telescopes then in use in Europe, and some astronomers 
professed to doubt their existence. But in the winter of 
1873 they were found with the twenty-six-inch Wash- 
ington telescope, which had just been completed, and 
were shown to move in exact accordance with the observa- 
tions of Lassell. 

The most remarkable feature of these bodies is that 
their orbits are nearly perpendicular to the orbit of the 
planet. The result is that there are two opposite points 
of the latter orbit where that of the satellite is seen edge- 



THE SATELLITES OF URANUS 229 

wise. When Uranus is near either of these points, we, 
from the earth, see the satelhtes moving as if swinging 
up and down in a north and south direction on each 
side of the planet, hke the bob of a pendulum. Then, as 
the planet moves on, the apparent orbits slowly open out. 
At the end of twenty years we see them perpendicularly. 
They then seem to us almost circular, but appear to close 
up again year after year as the planet moves on its 
course. The orbits were last seen edgewise in 1882, and 
will be again so seen about 1924. For several years to 
come the orbits are seen from a nearly perpendicular 
standpoint, which is the most favourable condition for 
observing the satellites. 

It is quite possible that continued observations of these 
bodies will yet enable the astronomer to reach some con- 
clusion to the hitherto unsolved problem of the rotation 
of Uranus on its axis. In the cases of Mars, Jupiter, and 
Saturn, the satellites revolve very nearly in the plane of 
the equators of the several planets to which they belong. 
If this is true of Uranus, it would follow that the equator 
of the planet was nearly perpendicular to its orbit, and 
that its north pole, at two opposite points in its orbit, 
would point almost exactly to the sun. Such being the 
case, the seasons would be vastly more marked than they 
are on our earth. Only on or near the equator of Uranus 
would a denizen of the planet see the sun every day. If 
he lived in middle latitudes there would be a period equal 
in length to five or ten of our years during which the sun 
would never reach his horizon. TL^hen, moving rapidly 
upwards, it would rise and set, giving him day and night, 



230 PLANETS AND THEIR SATELLITES 

but in time it would get so far up toward the north pole 
that it would never set during a period equal to that at 
Avhich it never rose. 

The fact that all the satellites revolve in almost ex- 
actly the same plane gives some colour to this view, but 
does not quite prove it, because it is not impossible that 
their planes are kept together by their mutual action. 
If, however, this is the case, and if the equator of Uranus 
does not coincide with the orbits, the latter will, in the 
course of centuries, undergo a change which our succes- 
sors will be able to determine. In this way they will be 
enabled to learn something of the equator and poles of 
Uranus, even if their telescopes are not powerful enough 
to afford any visual evidence on the subject. 



IX 

Neptune and its Satellite 

So far as yet known, Neptune is the outermost planet 
of our solar system. In size and mass it is not very 
different from Uranus, but its greater distance, 30 astro- 
nomical units, instead of 19.2, makes it fainter and 
harder to see. It is far below the limit of visibility by 
the naked eye, but quite a moderate-sized telescope would 
show it if one could only distinguish it from the numer- 
ous stars of similar brightness that stud the heavens. 
This needs astronomical appliances of a more refined 
and complex sort. 

The disk of Neptune is to be made out only with a 
telescope of considerable power. It is then seen to be of a 
bluish or leaden tint, perceptibly different from the sea- 
green of Uranus. Of course nothing can be known by 
direct observation about its rotation on its axis. Its spec- 
trum shows bands like those of Uranus, and it seems likely 
that the two bodies are much alike in their constitution. 

The discovery of Neptune in 1846 is regarded as one 
of the most remarkable triumphs of mathematical as- 
tronomy. Its existence was made known by its attraction 
on the planet Uranus before any other evidence had been 
brought out. The history of the circumstances leading 
to the discovery is so interesting that we shall briefly 
mention its main points. 



232 PLANETS AND THEIR SATELLITES 

History of the Discovery of Neptune 

During the first twenty years of the nineteenth cen- 
tury Bouvard, of Paris, an eminent mathematical astron- 
omer, prepared new tables of the motions of Jupiter, 
Saturn, and Uranus, then supposed to be the three outer- 
most planets. He took the deviations of these planets, 
produced by their attraction on each other, from the 
calculations of Laplace. He succeeded fairly well in 
fitting his tables to the observed motions of Jupiter and 
Saturn, but found that all his eff^orts to make tables that 
would agree with the observed positions of Uranus were 
fruitless. If he considered only the observations made 
since the discovery by Herschel, he could get along ; but 
no agreement could be obtained with those made previ- 
ously by Flamsteed and Lemonnier, when the planet was 
supposed to be a fixed star. So he rejected these old 
observations, fitted his orbit into the modern ones, and 
published his tables. But it was soon found that the 
planet began to move away from its calculated position, 
and astronomers began to wonder what was the matter. 
It was true that the deviation, measured by a naked eye 
standard, was very small; in fact, if there had been two 
planets, one in the real and one in the calculated position, 
the naked eye could not have distinguished them from 
a single star. But the telescope would have shown them 
well separated. 

Thus the case stood until 1845. At that time there 
lived in Paris a young mathematical astronomer, Lever- 
rier, who had already made a name in his science, having 



DISCOVERY OF NEPTUNE 

communicated to the Academy of Sciences some re- 
searches which gave Arago a very high opinion of his 
abihties. Arago called his attention to the case of 
Uranus and suggested that he should investigate the sub- 
ject. The idea occurred to Leverrier that the deviations 
were probably caused by the attraction of an unknown 
planet outside of Uranus. He proceeded to calculate in 
what orbit a planet should move to produce them, and 
laid his result before the Academy of Sciences in the 
summer of 1846. 

It happened that, before Leverrier commenced his 
work, an English student at the University of Cam- 
bridge, Mr. John C. Adams, had the same idea and set 
about the same work. He got the result even before 
Leverrier did, and communicated it to the Astronomer 
Royal. Both computers calculated the present position 
of the unknown planet, so that, were it possible to dis- 
tinguish it from a fixed star, it would only have been 
necessary to search in the region indicated in order to 
find the planet. Unfortunately, however, Airy was in- 
credulous as to the matter, and did not think the chance 
of finding the planet sufficient to go through the labori- 
ous operation of a search until his attention was attract- 
ed by the prediction of Leverrier, and the close agree- 
ment between the two computers was remarked. 

The problem of finding the planet was now taken up. 
Very thorough observations were made upon the stars in 
the region by Professor Challis at the Cambridge Obser- 
vatory. I must explain that, as it was not easy with the 
imperfect instruments of that time to distinguish so 



234 PLANETS AND THEIR SATELLITES 

small a planet from the great number of fixed stars 
which studded the heavens around it, it was necessary 
to proceed by determining the position of as many stars 
as possible several times, in order that, by a comparison 
of the observations, it could be determined whether any 
of them had moved out of its place. 

While i\Ir. Challis was engaged in tliis work it oc- 
curred to Leverrier that the astronomers of Berlin were 
mapping the heavens. He therefore wrote to Encke, the 
director of the Berlin Observatory, suggesting that they 
should look for the planet. Now it happened that the 
Berlin astronomers had just completed a map of that 
part of the sky in which the planet was located. So, on 
the very evening after the letter was received, they took 
the map to the telescope and proceeded to search about 
to see if any object was seen in the telescope which was 
not on the map. Such an object was very soon found, 
and, by comparing its position with that of the stars 
around it, it seemed to have a slight motion. But Encke 
was very cautious and waited for the discovery to be con- 
firmed on the night following. Then it was found to 
have moved so much that no doubt could remain, and he 
wrote Leverrier that the planet actually existed. 

When this news reached England, Professor Challis 
proceeded to examine his own observations, and found 
that he had actually observed the planet on two occa- 
sions. Unfortunately, however, he had not reduced and 
compared his observations, and so failed to recognise the 
object until after it had been seen at Berlin. 

The question of the credit due to Adams gave rise to 



THE SATELLITE OF NEPTUNE 235 

much controversy, Arago in France claiming that, in the 
history of the affair, the name of Adams should not even 
be mentioned — the whole credit should go to Leverrier. 
This he did on the principle that it was not the person 
who first did a thing, but he who first published it, who 
should receive the credit. But the English claimed that, 
as Adams had actually preceded Leverrier and, if he had 
not printed his paper, had at least communicated it to 
public authorities, and had enabled Challis to see, al- 
though not to recognise, the planet, he should get his due 
share of credit. The whole question thus raised was one 
of honour, and subsequent astronomers have taken the 
very proper course of honouring both men all they could 
for so wonderful a work. 

The Satellite of Neptune 

Of course the newly found j)lanet was observed by 
astronomers the world over. The result was that Mr. 
Lassell soon found that Neptune was accompanied by 
a satellite. This object was observed at the few observa- 
tories then possessing telescopes of sufficient power to 
make it out. Its time of revolution was found to be 
nearly six days. v 

The most curious feature of this satellite is that, con- 
trary to the rule in the case of all the bodies of the solar 
system except Uranus, it moves from east toward west. 
In the case of Uranus we cannot consider the motion as 
being east or west, we should rather call it a north and 
south motion. 

It would be very interesting to know whether the 



286 PLANETS AND THEIR SATELLITES 

planet Neptune revolves on its axis in the same direction 
as the satellite moves. But this cannot be determined, 
because it is so distant and its disk so faint and diffuse 
that no markings can be detected upon it. Indeed, if we 
reflect that the rotation of a planet so near us as Venus 
has never been certainly determined, we may easily see 
how hopeless is the prospect of determining that of 
Neptune. 

But, in spite of this, there is remarkable evidence that 
the planet has a rapid rotation. It is found that the 
orbit of the satellite is very slowly changing its position 
from year to year. During the half century since obser- 
vations commenced, this change amounts to several de- 
grees. The only way in which it can be accounted for 
is by supposing that Neptune, like the earth and the other 
rapidly rotating planets, is an oblate ellipsoid, and that 
the plane of the planet's equator does not coincide with 
that of the orbit of the satellite. In time the astronomer 
will be able to learn from this motion the position of the 
poles and equator of the planet Neptune, but this may 
require a century of observation, or even several centuries. 



X 

How THE Heavens are Measured 

Distances in the heavens may be determined by a 
method similar to that employed by an engineer in de- 
termining the distance of an inaccessible object — say a 
mountain peak. Two points, A and B, are taken as a 
base line from which to measure the distance of a third 
point, C. Setting up his instrument at A, the engineer 
measures the angle between B and C. Setting it up at 
B he measures the angle between A and C. Since the 
sum of the three angles of a triangle is always one hun- 
dred and eighty degrees, the angle at C is found by 




of the Distance of an Inaccessible Object by Triangrilation. 



subtracting the sum of the angles at A and B from that 
quantity. It will readily be seen that the angle at C 
is that subtended by the base line as it would appear if 
viewed by an observer at C. Such an angle is, in a 
general way, called a parallax. It is the difference of 
direction of the point C as seen from the points A and B. 
It will readily be seen that, with a given base line, 



£38 PLANETS AND THEIR SATELLITES 

the greater the distance of the object the less will be its 
parallax. At a sufficiently great distance the latter will 
be so small that the observer cannot get any evidence of 
it. To all appearance the lines B C and A C will then 
have the same direction. The distajice at which the 
parallax cannot be made out depends, of course, on the 
accuracy of the measurement, and the length of the 
base line. 

The moon being the nearest of all the heavenly bodies 
has the largest parallax. Its distance can therefore be 
determined with the greatest precision by measurement. 
Even Ptolemy, who lived only one or two centuries after 
Christ, was able to make an approximate measure of the 
distance of the moon. But the parallax of a planet is so 
small that it can be determined only with the most refined 
instruments. 

The ends of the base line used in the determination 
may be any two points on the earth's surface — say the 
observatories of Greenwich and the Cape of Good Hope. 
In the case of the transits of Venus, which we have al- 
ready described, there were a number of different sta- 
tions at various points on the earth's surface, from which 
the direction of Venus at the beginning and end of its 
transit could be inferred. This method of determining 
distances is called triangulation. 

The idea of a triangulation, as thus set forth, gives an 
understanding only of the general principle involved in 
the problem. One can readily see that it would be out 
of the question for two observers in distant parts of the 
earth to get the exact direction of a planet at the same 



THE MOTION OF LIGHT 239 

moment of time. The actual determination of the paral- 
lax requires a combination of observations too complex 
to be set forth in the present book, but the fundamental 
principle is that just explained. 

In order to get the dimensions of the whole solar sys- 
tem, it is only necessary to know the distance of any one 
planet from us at any given moment. The orbits and 
motions of all the planets are mapped down with the 
greatest possible exactness, but with the map before us 
we are in the position that one would be who had a very 
exact map of a country, only there was no scale of miles 
upon it. So he would be unable to measure the distance 
from one point to another on his map until he knew the 
scale. It is the scale of our map of the solar system 
which the astronomer stands in need of and which he has 
not, even with the most refined instruments, yet been able 
to determine as accurately as he could wish. 

The fundamental unit aimed at is that already de- 
scribed — the mean distance of the earth from the sun. 
Measures of parallax are by no means the only method of 
determining this distance. Within the last fifty years 
other methods have been developed, some of which are 
fully as accurate as the best measures of parallax, per- 
haps even more so. 

Measurement by the Motion of Light 

One of the most simple and striking of these methods 
makes use of the velocity of light. By observations of 
Jupiter's satellites, made when the earth was at different 
points of its orbit, it has been found that light passes 



240 PLANETS AND THEIR SATELLITES 

over a distance equal to that of the earth from the sun 
in about eight minutes twenty seconds, or five hundred 
seconds. This determination has been more accurately 
made in another way by the aberration of the stars. 
This is a slight change in their position due to the com- 
bined motion of the earth and the ray of light by which 
we see the star. By accurate observations on the aberra- 
tion, it is found that light travels from the earth to the 
sun in almost exactly 499.6 seconds. It follows that if we 
can find how far light will travel in one second, we can 
determine the distance of the sun by multiplying the re- 
sult by 499.6. The measurement of the velocity of light 
is one of the most difficult problems in physics, as it re- 
quires the measurement of intervals of tim.e only a few 
millionths of a second in duration. Those who are inter- 
ested in the subject will see the method of doing this 
explained in special treatises ; at present it is sufficient to 
say that light is found to travel 299,860 kilometres, or 
186,300 miles in a second. Multiply this by 499.6 and 
we have 93,075,480 miles for the distance of the sun from 
the earth. 

Measurement hy the Sun's Gramtation 

A third method rests on the measures of the sun's 
gravitation upon the moon. One effect of this is that, 
as the moon performs its monthly revolution round the 
earth, it is at its first quarter a little more than two 
minutes behind its average position, to which it catches 
up at full moon, and passes ; so that at last quarter it is 
two minutes ahead of the mean position. Toward new 



THE SUN'S GRAVITATION 241 

moon it falls behind again to the average place. Thus 
a slight swing goes on in unison with the moon's mo- 
tion around the earth. The amount of this swing is 
inversely proportional to the distance of the sun. Hence, 
by measuring this amount, its distance may be deter- 
mined. As in other astronomical measurements, the 
difficulty of the determination is very great. A swing 
like this is very hard to measure without error; more- 
over, the problem of determining just how much swing 
the sun would produce at a given distance is one of the 
difficult problems of celestial mechanics, which has not 
yet been solved so satisfactorily as to leave no doubt 
whatever on the result. 

The fourth method also rests on gravitation. If we 
only knew the exact relation between the mass of the 
earth and that of the sun; that is to say, if we could 
determine precisely how many times heavier the sun is 
than the earth, we could compute at what distance the 
earth must be placed from the sun in order to revolve 
around it in one year. The only difficulty, therefore, is 
to weigh the earth against the sun. This is most exactly 
done by finding the change in the position of the orbit 
of Venus produced by the earth's attraction. By com- 
paring the positions of the orbit of Venus by its transits 
in 1761, 1769, 1874, and 1882, it is found that the orbit 
has a progressive motion, indicating that the mass of 
the sun is 332,600 times that of the earth and moon com- 
bined. Thus we are enabled to compute the distance of 
the sun by still another method. 



2n PLANETS AND THEIR SATELLITES 

Results of Measurements of the Sun's Distance 

We have described four methods of making this fun- 
damental determination in astronomy, and in order that 
the reader may see just what degree of certainty and 
precision astronomical theory and measurements have 
reached, we give the separate results of these methods. 
The first column shows the parallax of the sun, which 
is the quantity actually used by astronomers. It is the 
same thing as the angle under which the equatorial 
radius of the earth would be seen by an observer at the 
distance of the sun from us. This is followed by the 
accompanying distance in miles. 



Measures of parallax 8.800 

Velocity of light 8.778 

Motion of moon 8. 784 

Mass of the earth 8.762 



Dist. 92,908,000 miles 

" 93,075,480 " 

" 92,958,000 " 

" 93,113,000 " 



The difference between these results is no greater than 
the liability of error wherever mathematical demonstra- 
tions and instrumental measurements of such extreme 
minuteness and complexity as these are required. From 
the close agreement between results reached by methods 
so widely different in their principles, we have a striking 
proof of the correctness of the astronomical views of the 
universe. Yet discrepancies exceeding a hundred thou- 
sand miles will not be tolerated by astronomers longer 
than is absolutely necessary. 



XI 

Gravitation and the Weighing of the Planets 

No work of the human intellect farther transcends 
what would seem possible to the ordinary thinker than the 
mathematical demonstrations of the motions of the heav- 
enly bodies under the influence of their mutual gravita- 
tion. We have learned something of the orbits of the 
planets round the sun; but the following of the orbit is 
not the fundamental law of the planet's motion ; the lat- 
ter is determined by gravitation alone. This law, as 
stated by Newton, is so comprehensive that nothing can 
be added. The law that every particle of matter in the 
universe attracts every other particle, with a force which 
varies inversely as the square of the distance between 
them, is the only law of nature which, so far as we know, 
is absolutely universal and invariable in its action. All 
the other processes of nature are in some way varied or 
modified by heat and cold, by time or place, by the pres- 
ence or absence of other bodies. But no operation that 
man has ever been able to perform on matter changes its 
gravitation in the slightest. Two bodies gravitate by 
exactly the same amount, no matter what we do with 
them, no matter what obstacles we interpose between 
them, no matter how fast they move. All other natural 
forces admit of being investigated, but gravitation does 
not. Philosophers have attempted to explain it, or to 



244 PLANETS AND THEIR SATELLITES 

find some cause for it, but nothing has yet been added 
to our knowledge by these attempts. 

The motions of the planets are governed by their 
gravitation. Were there only a single planet moving 
round the sun it would be acted on by no force but the 
sun's attraction. By purely mathematical calculation it 
is shown that such a planet would describe an ellipse, 
having the sun in one focus. It would keep going round 
and round in this ellipse forever. But in accordance with 
the law, the planets must gravitate towards each other. 
This mutual gravitation is far less than that of the sun, 
because in our solar system the planets are of much 
smaller mass than the central body. In consequence of 
this mutual attraction the planets deviate from the 
ellipse. Their orbits are very nearly, but not exactly, 
ellipses. Still, the problem of their motion is one of pure 
mathematical demonstration. It has occupied the ablest 
mathematicians of the world since the time of Newton. 
Every generation has studied and added to the work of 
the preceding one. One hundred years after Newton, 
Laplace and Lagrange showed that the ellipses near 
which the planets move gradually change their form and 
position. These changes can be calculated thousands, 
tens of thousands, or even hundreds of thousands of 
years in advance. Thus it is known that the eccentricity 
of the earth's orbit round the sun is now slightly 
diminishing, and that it will continue to diminish for 
about forty thousand years. Then it will increase so 
that in the course of many thousands more of years it 
will be greater than it now is. The same is true of all 



GRAVITATION AND WEIGHING 245 

the planets. Their orbits gradually change their form 
back and forth through tens of thousands of years, like 
"great clocks of eternity which count off ages as ours 
count off seconds." The ordinary reader would be justi- 
fied in some incredulity as to the correctness of these 
predictions for thousands of years to come, were it not 
for the striking precision with which the motions of the 
planets are actually predicted by the mathematical 
astronomer. This precision is reached by solving the 
very difficult problem of determining the effect of each 
planet on the motions of all the other planets. We might 
predict the motions of these bodies by assuming that each 
of them moves round the sun in a fixed ellipse, which, 
as I have just said, would be the case if it were not 
attracted by any other body. Our predictions would 
then, from time to time, be in error by amounts which 
might amount to large fractions of a degree ; perhaps, 
in the course of a long time, to even more. To form an 
idea of this error we may say that one degree is the angle 
at which we see the breadth of an ordinary window frame 
at the distance of a hundred yards. The planet might 
then be predicted as in a line with one side of such a 
frame when in reality it would be at the other side or in 
the middle of the window. 

But, taking account of the attraction of all the other 
planets, the prediction is so exact that the refined obser- 
vations of astronomy hardly show any appreciable de- 
viation. If we should mark on the side of a distant house 
a row of a hundred points, each apparently as far from 
the other as the average error of these predictions, the 



246 PLANETS AND THEIR SATELLITES 

whole row would seem to the naked eye as a single point. 
The history of the discovery of Neptune, which was 
mentioned in the preceding chapter, affords the most 
striking example that we possess of the certainty of these 
predictions. 

How the Planets are Weighed 

I shall now endeavour to give the reader some idea of 
the manner in which the mathematical astronomer reaches 
these wonderful results. To make them, he must, of 
course, know the pull each planet exerts upon the others. 
This is proportional to what the physicist and astrono- 
mer call the mass of the attracting planet. This word 
means quantity of matter, and around us on the surface 
of the earth, it has nearly the same meaning as the word 
weight. We may therefore say that, when the astrono- 
mer determines the mass of a planet, he is weighing it. 
He does this on the same principle by which the butcher 
weighs a ham in the spring balance. When the butcher 
picks the ham up he feels a pull of the ham toward the 
earth. When he hangs it on the hook, this pull is trans- 
ferred from his hand to the spring of the balance. The 
stronger the pull the farther the spring is pulled down. 
What he reads on the scale is the strength of the pull. 
You know that this pull is simply the attraction of the 
earth on the ham. But, by a universal law of force, the 
ham attracts the earth exactly as much as the earth does 
the ham. So what the butcher really does' is to find how 
much or how strongly the ham attracts the earth, and 
he calls that pull the weight of the ham. On the same 



HOW THE PLANETS ARE WEIGHED 247 

principle, the astronomer finds the weight of a body by 
finding how stronq* is its attractive pull on some other 
body. 

In applying this principle to the heavenly bodies, you 
meet at once a difficulty that looks insurmountable. You 
cannot get up to the heavenly bodies to do your weigh- 
ing ; how then will you measure their pull ? I must begin 
the answer to this question by explaining more exactly 
the difference between the weight of a body and its mass. 
The weight of obj ects is not the same all over the world ; 
a thing which weighs thirty pounds in New York would 
weigh an ounce more than thirty pounds in a spring 
balance in Greenland, and nearly an ounce less at the 
equator. This is because the earth is not a perfect sphere, 
but a little flattened. Thus weight varies with the place. 
If a ham weighing thirty pounds were taken up to the 
moon and weighed there, the pull would only be five 
pounds, because the moon is so much smaller and lighter 
than the earth. But there would be just as much ham 
on the moon as on the earth. There would be another 
weight of the ham on the planet Mars, and yet another 
on the sun, where it would weigh some eight hundred 
pounds. Hence, the astronomer does not speak of the 
weight of a planet, because that would depend on the 
place where it was weighed ; but he speaks of the mass of 
the planet, which means how much planet there is, no 
matter where you might weigh it. 

At the same time we might, . without any inexactness, 
agree that the mass of a heavenly body should be fixed by 
the weight it would have at some place agreed upon, say 



248 PLANETS AND THEIR SATELLITES 

New York. As we could not even imagine a planet at 
New York, because it may be larger than the earth itself, 
what we are to imagine is this : Suppose the planet could 
be divided into a million million million equal parts, and 
one of these parts brought to New York and weighed. 
We could easily find its weight in pounds or tons. Then 
multiply this weight by a milhon million million and we 
shall have a weight of the planet. This would be what 
the astronomers might take as the mass of the planet. 

With these explanations, let us see how the weight of 
the earth is found. The principle we apply is that round 
bodies of the same specific gravity attract small objects 
on their surface with a force proportional to the diameter 
of the attracting body. For example, a body two feet 
in diameter attracts twice as strongly as one of a foot, 
one of three feet three times as strongly, and so on. Now, 
our earth is about forty million feet in diameter ; that is, 
ten million times four feet. It follows that if we made 
a little model of the earth four feet in diameter, having 
the average specific gravity of the earth, it would attract 
a particle with one ten-millionth part of the attraction 
of the earth. We have shown in our chapter on the earth 
how the attraction of such a model has actually been 
measured, with the result of showing that the total mass 
of the earth is five and one half times that of an 
equal bulk of water. Thus this mass becomes a known 
quantity. 

We come now to the planets. I have said that the 
mass or weight of a heavenly body is determined by its 
attraction on some other body. There are two ways in 



HOW THE PLANETS ARE WEIGHED 249 

which the attraction of a planet may be measured. One 
is by its attraction on the planets next to it, causing 
them to deviate from the orbits in which they would move 
if left to themselves. By measuring the deviations, we 
can determine the amount of the pull, and hence the mass 
of the planet. 

The reader will readily understand that the mathe- 
matical processes necessary to get a result in this way 
must be very delicate and complicated. A much simpler 
method can be used in the case of those planets which 
have satellites revolving round them, because the attrac- 
tion of the planet can be determined by the motions of 
the satellite. The first law of motion teaches us that a 
body in motion, if acted on by no force, will move in a 
straight line. Hence, if we see a body moving in a curve, 
we know that it is acted on by a force in the direction 
toward which the motion curves. A familiar example is 
that of a stone thrown from the hand. If the stone were 
not attracted by the earth it would go on forever in the 
line of throw, and leave the earth entirely. But under 
the attraction of the earth it is drawn down and down, 
as it travels onward, until finally it reaches the ground. 
The faster the stone is thrown, of course, the farther it 
will go, and the greater will be the sweep of the curve of 
its path. If it were a cannon ball, the first part of the 
curve would be nearly a right line. If we could fire a 
cannon ball horizontally from the top of a high moun- 
tain with a velocity of five miles a second, and if it were 
not resisted by the air, the curvature of the path would 
be equal to that of the surface of our earth, and so the 



250 PLANETS AND THEIR SATELLITES 

ball would never reach the earth, but would revolve round 
it like a little satellite in an orbit of its own. Could this 
be done the astronomer would be able, knowing the 
velocity of the ball, to calculate the attraction of the 
earth. The moon is a satellite, moving like such a ball, 
and an observer on Mars would be able, by measuring 
the orbit of the moon, to determine the attraction of the 
earth as well as we determine it by actually observing 
the motion of falling bodies around us. 

Thus it is that when a planet like Mars or Jupiter has 
satellites revolving around it, astronomers on the earth 
can observe the attraction of the planet on its satellites 
and thus determine its mass. The rule for doing this is 
very simple. The cube of the distance between the planet 
and satellite is divided by the square of the time of revo- 
lution. The quotient is a number which is propor- 
tional to the mass of the planet. The rule applies 
to the motion of the moon round the earth and of 
the planets round the sun. If we divide the cube 
of the earth's distance from the sun, say ninety-three 
millions of miles, by the square of three hundred and 
sixty-five and a quarter, the days in a year, we shall gst 
a certain quotient. Let us call this number the sun- 
quotient. Then, if we divide the cube of the moon's dis- 
tance from the earth by the square of its time of revolu- 
tion, we shall get another quotient, which we may call 
the earth-quotient. The sun-quotient will come out 
about three^ hundred and thirty thousand times as large 
as the earth-quotient. Hence it is concluded that the 
mass of the sun is three hundred and thirty thousand 



HOW THE PLANETS ARE WEIGHED 251 

times that of the earth; that it would take this number 
of earths to make a body as heavy as the sun. 

I give this calculation to illustrate the principle; it 
must not be supposed that the astronomer proceeds ex- 
actly in this way and has only this simple calculation to 
make. In the case of the moon and earth, the motion and 
distance of the former vary in consequence of the attrac- 
tion of the sun, so that their actual distance apart is a 
changing quantity. So what the astronomer actually 
does is to find the attraction of the earth by observing 
the length of a pendulum which beats seconds in various 
latitudes. Then by very delicate mathematical processes 
he can find with great exactness what would be the time 
of revolution of a small satellite at any given distance 
from the earth, and thus can get the earth-quotient. 

But, as I have already pointed out, we must, in the 
case of the planets, find the quotient in question by means 
of the satellites ; and it happens, fortunately, that the 
motions of these bodies are much less changed by the at- 
traction of the sun than is the motion of the moon. Thus, 
when we make the computation for the outer satellite of 
Mars, we find the quotient to be g^Qgg^gQQ that of the 
sun-quotient. Hence we conclude that the mass of Mars 
i^ 3,093,500 ^^^^ ^^ ^^^ s^^- ^y ^^^ corresponding quo- 
tient, the mass of Jupiter is found to be about y^^oTi 
that of the sun ; Saturn, 37^ ; Uranus, 22,700 ' Neptune, 



19,500 

I have set forth only the great principle on which the 
astronomer has proceeded for the purpose in question. 
The law of gravitation is at the bottom of all his work. 



252 PLANETS AND THEIR SATELLITES 

The effects of this law require mathematical processes 
which it has taken two hundred years to bring to their 
present state, and which are still far from perfect. The 
measurement of the distance of a satellite is not a job 
to be done in an evening; it requires patient labor ex- 
tending through months and years, and then is not as 
exact as the astronomer would wish. He does the best 
he can and must be satisfied with the result until he can 
devise an improvement on his work, which he is always 
trying to do with varying success. 



PART V 
COMETS AND METEORIC BODIES 



I 

Comets 

Comets differ from the heavenly bodies which we have 
hitherto studied in their pecuhar aspects, their eccentric 
orbits, and the rarity of their appearance. Some mystery 
still surrounds the question of their constitution, but this 
does not detract from the interest of the phenomena 
which they present. When one of these objects is care- 
fully examined we find it to embody three features which, 
however, are not separate and distinct, but merge into 
each other. 

First we have what, to the naked eye, appears to be 
a star of greater or less brilliancy. This is called the 
nucleus of the comet. 

Surrounding the nucleus is a cloudy nebulous mass, 
like a little bunch of fog, shading off very gradually to- 
ward the edge, so that we cannot exactly define its bound- 
ary. This is called the coma (Latin for hair). Nucleus 
and coma together are called the head of the comet, 
which looks like a star shining through a patch of mist 
or fog. 

Stretching away from the comet is the tail, which may 
be of almost any length. In small comets the tail may be 
ever so short, while in the greatest it stretches over a long 
arc of the heavens. It is narrow and bright near the head 
of the comet and grows wider and more diffuse as it 



256 COMETS AND METEORIC BODIES 

recedes from the head. It is therefore always more or 
less fan-shaped. Toward the end it fades away so gradu- 
ally that it is impossible to say how far the eye can 
trace it. 

Comets differ enormously in brightness, and, notwith-,. 
standing the splendid aspect which the brighter ones as- 
sume, the great majority of these objects are quite invis- 
ible to the naked eye. Such are called telescopic comets. 
There is, however, no broad distinction to be drawn be- 
tween a telescopic comet and a bright one, there being 
a regular range of brightness from the faintest of these 
objects to the most brilliant. Sometimes a telescopic 
comet has no visible tail; this, however, is the case only 
when the object is extremely faint. Sometimes, also, the 
nucleus is almost wholly wanting. In such a case all 
that can be seen is a small hairy mass, like a very thin 
cloud, which may be a little brighter in the centre. 

From the historical records it would appear that from 
twenty to thirty comets visible to the naked eye gener- 
ally appear in the course of a century. But when the 
telescope was employed in sweeping the heavens it was 
found that these objects were more numerous than had 
been supposed. Quite a number are now found every 
year by diligent observers. Doubtless the number de- 
pends very largely on accident, as well as on the skill 
applied in the search. Sometimes the same comet will be 
found independently by several observers. The credit is 
then given to the one who first accurately fixes the posi- 
tion of the comet at a given time, and telegraphs the fact 
to an observatory. 



ORBITS OF COMETS 



257 



Orbits of Comets 

Soon after the invention of the telescope it was found 
that comets resembled the planets in moving in orbits 
around the sun. Sir Isaac Newton showed that their 
motions were ruled by the sun's gravitation in the same 
way as the motions of the planets. The great difference 
was that, instead of the orbits being nearly circular, like 
those of the planets, they were so elongated that, in most 
cases, it could not be determined where the aphelion, or 
farther end, was. As many of our readers may desire 
an exact statement of the nature of cometary orbits, and 
the laws governing them, we shall enter into some 
explanations of the subject. 

It was shown by Newton 
that a body moving under 
the influence of the sun's at- 
traction would always de- 
scribe a conic section. This 
curve is of three kinds, an 
ellipse, a parabola, and a 
hyperbola. The first, as we 
all know, is a closed curve 
returning into itself. But 
the parabola and the hyper- 
bola are not such ; each of them extends out without end 
in two branches. In the case of the parabola these two 
branches approach more nearly to having the same direc- 
tion as we get out farther, but in the case of the hyper- 
bola they always diverge from each other. 




Fig. 45. 



-Parabolic Orbit of a 
Comet. 



258 COMETS AND METEORIC BODIES 

Having these curves in mind, let us imagine the earth 
to leave us hanging in space at some point of its orbit, 
our planet pursuing its course without us, until, at the 
end of a year, it returns to pick us up again. During 
the interval of its absence we, hanging in mid-space, 
amuse ourselves by firing oif balls to perform their revo- 
lutions around the sun like little planets. The result will 
be that all the balls we send off with a velocity less than 
that of the earth, that is to say, less than eighteen and 
six tenths miles per second, will move around the sun in 
closed orbits, smaller than the orbit of the earth, no mat- 
ter what direction we send them in. A very simple and 
curious law is that these orbits will always have the same 
period if the velocity is the same. All the balls sent with 
the velocity of the earth will be one year in making their 
revolution and will, therefore, come together, at the point 
from which they started, at the same moment. If the 
velocity exceeds eighteen and six tenths miles a second, the 
orbit will be larger than that of the earth and the period 
of revolution will be longer the greater the velocity. 
With a speed exceeding about twenty-six miles a second, 
the attraction of the sun could never hold in the ball, 
which would fly away for good in one of the branches of 
a hyperbola. This would happen no matter in what 
direction we threw the object. There is, therefore, at 
every distance from the sun, a certain limiting velocity 
which, if a comet exceeds, it will fly off from the sun 
never to return; while, if it falls short, it will be sure to 
get back at some time. 

The nearer we are to the sun, the greater is this limit- 



ORBITS OF COMETS ^59 

ing velocity. It varies inversely as the square root of 
the distance from the sun, hence, four times away from 
the sun, it is only half as great. The rule for finding 
the limiting velocity at any point in space is very simple. 
It is to take the speed of a planet passing through that 
point in a circular orbit, and multiply it by the square 
root of 2. This is 1.414. . . . 

It follows that if the astronomer, by means of his ob- 
servations, can find the velocity with which a comet is 
passing a known point of its orbit, he can determine the 
distance to which it will fly from the sun and the period 
of its return. By a careful comparison of observation 
made during the whole period of visibility of the comet 
he can generally reach a definite conclusion on the 
subject. 

It is a curious fact that no comet has yet been seen of 
which the speed certainly exceeds the limit which we have 
described. It is true that, in many cases, a slight excess 
has been calculated from the observations, but this excess 
was no greater than might result from the necessary 
errors of observations on bodies of this kind. Commonly 
the speed is so near the limit that it is impossible to say 
whether it exceeds it or not. It is then certain that the 
comet will fly out to an immense distance, not returning 
for hundreds, thousands, or tens of thousands of years. 
There are also cases in which the speed of the comet is 
found to be less than the limit by a considerable amount. 
Such comets complete their revolutions in shorter periods 
and are called periodic comets. 

So far as we know, the history of the motion of the 



260 COMETS AND METEORIC BODIES 

large majority of the comets is this. They appear to 
us as if falling toAvard the sun from some great distance, 
we know not what. If a comet fell exactly toward the 
sun, it would fall into it, but this is a case which has not 
been known to occur and which, for reasons to be ex- 
plained later, cannot be expected ever to occur. As it 
approaches the sun, it acquires greater and greater 
velocity, speeds around the central body in a great curve, 
and, by the centrifugal force thus generated, flies ofl* 
again, returning to the abyss of space nearly in the 
direction from which it came. 

Owing to the faintness of these objects they are visible, 
even in powerful telescopes, only in that part of their 
orbit which is comparatively near the sun. This is what 
makes it so difficult in many cases to determine the exact 
period of a comet which has only been seen once. 

Halle y^s Comet 

The first of these objects which was found to return 
in a regular period is celebrated in the history of astron- 
omy under the name of Halley's comet. It appeared in 
August, 1682, and was observed for about a month, when 
it disappeared from view. Halley was able, from the ob- 
servations made upon it, to compute the position of the 
orbit. He found that the latter was in the same position 
as that of a bright comet observed by Kepler in 1607. 

It did not seem at all likely that two comets should 
move precisely in the same orbit. Halley therefore 
judged that the real orbit was an ellipse, and that the 
comet had a period of about seventy-five years. If this 



HALLEY'S COMET 261 

were the case, it should have been visible at intervals of 
about seventj-five years in the past. 

So he subtracted this period from the several dates in 
order to determine whether any comets were recorded. 
Subtracting seventy -five from 1607 we have 1532. He 
found that a comet had actually appeared in 1531, which 
he had reason to believe was moving in the same orbit. 
Again subtracting seventy-five from this year we have 
the 3 ear 11^56. A comet really did appear in 1456, which 
spread such horror throughout Christendom that Pope 
Calixtus III ordered prayers to be offered for protection 
against the comet as well as against the Turks, who were 
at war against Europe. It is probable that the myth 
of "the Pope's Bull against the comet" refers to this 
circumstance. 

Other possible appearances of the comet were found in 
past history, but Halley was not able to identify the 
comet with exactness, owing to the absence of any pre- 
cise description of the body. But the four well-observed 
dates, 1456, 1531, 1607, and 1682, afforded ample 
ground for predicting that the comet would again return 
to the sun about 1758. Clairaut, one of the most eminent 
mathematicians then in France, was able to calculate what 
effect would be produced by the action of Jupiter and 
Saturn on the period of the comet. He found that this 
action would so delay its return that it would not reach 
perihelion until the spring of 1759. It appeared accord- 
ing to the prediction, and actually passed perihelion on 
March twelfth of that year. 

The next predicted return was in 1835. Several 



S62 COMETS AND METEORIC BODIES 

mathematicians now made computations of the effect of 
the planets in changing its period. So exact was their 
work that two of them hit the time within five days : Pro- 
fessor Rosenberger assigned November eleventh as the 
date of return, and Pontecoulant predicted it for Novem- 
ber thirteenth. It actually passed perihelion on November 
sixteenth. After being observed for several months it 
disappeared from view and has not since been seen. But 
so exact is astronomical science that an astronomer could, 
at any time during the intervening interval, have pointed 
his telescope exactly at the object, after making the 
necessary calculations to determine its position. 

Its next return is now approaching, but the exact date 
has not yet been computed. It will probably be some 
time between 1910 and 1912. 

Comets which have Disappeared 

The most striking discovery of a comet after Halley 
announced the one which bears his name, was made 
by the French astronomer Lexell, in June, 1770. The 
object soon became visible to the naked eye. On laying 
down the orbit in which it moved, it w^as found, to the 
surprise of astronomers, that the orbit was an ellipse, 
with a period of only about six years. Its return was, 
therefore, confidently predicted, but it never reappeared. 
The cause was, however, speedily discovered. When it 
returned at the end of six years, it was on the opposite 
side of the sun, and therefore could not be seen. Passing 
out to complete its revolution, it was found by calculation 
that it must have gone into the immediate neighbourhood 



COMETS WHICH HAVE DISAPPEARED 263 

of the planet Jupiter, which, by its powerful attraction, 
started the comet off into some new orbit, so that it never 
again came within reach of the telescope. This, also, 
explained why the comet had not been seen before. Three 
years before Lexell found it, it had come from the neigh- 
bourhood of the planet Jupiter, which had thrown it into 
an orbit different from its former one. Thus the giant 
planet of our system had, so to speak, given the comet a 
pull about 1767 so that it should pass into the immediate 
neighbourhood of the sun, and having allowed it to make 
two revolutions around the sun, again encountered it in 
1779, and gave it a new swing off, no one knows where. 
Since that time twenty or thirty comets, found to be 
periodic, have been observed, most, but not all of them, 
at two or more returns. 

The most remarkable fact brought out by the study 
of these objects has been that they do not appear to be 
of seemingly infinite duration, like the planets, but are, 
as a general rule, subject to dissolution and decay, like 
living beings. The most curious case of a comet being 
completely disintegrated is that of Biela's comet. This 
was first observed in 1772, but was not known to be peri- 
odic. It was again seen in 1805, and again the astrono- 
mer did not notice the identity of the orbit in which it was 
moving with that of the comet of 1772. In 1826 it was 
discovered a third time, and now, on computing the orbit 
by the improved methods which had been invented, its 
identit}^ with the former comets was brought out. The 
time of revolution was fixed at six and two thirds years. 
It should, therefore, appear in 1832 and 1839. But on 



264 COMETS AND METEORIC BODIES 

these returns the earth was not in a position to admit of 
its being seen. Toward the end of 18-f5- it again ap- 
peared and was observed in November and December. 
In January, 1846, as it came nearer to the earth and sun, 
it was found to have separated into two distinct bodies. 
At jfirst the smaller of these was quite faint, but it seemed 
to increase in brightness until it became equal to the other. 

The next return was in 1852. The two bodies were 
then found to be far more widely separated than before. 
In 1846 their distance apart was about two hundred 
thousand miles ; in 1852 more than a million miles. The 
last observations were made in September, 1852. Al- 
though since that time the comet should have completed 
seven revolutions, it has never again been seen. From the 
former returns it was possible to compute the position 
where it should appear with a good deal of precision, and 
from its non-appearance we conclude that it has been 
completely disintegrated. We shall, in the next chapter, 
learn a little more about the matter which composed it. 

Two or three comets have disappeared in the same way. 
They were observed for one or more revolutions, growing 
fainter and more attenuated on each occasion, and finally 
became completely invisible. 

Encke's Comet 

Of the periodic comets the one that is most frequently 
and regularly observed bears the name of Encke, the 
German astronomer who first accurately determined its 
motion. Its first discovery was made in 1786, but, as 
was often the case then, its orbit could not at first be 



ENCKE'S COMET 265 

determined. It was again seen in 1795 by Miss Caroline 
Herschel. It was found again in 1805 and 1818. Not 
until the latter date was the accurate orbit determined, 
and then the periodic character of the comet and its iden- 
tity with the comet observed in previous years was 
established. 

Encke now found the period to be about three years 
and one hundred and ten days, varying a little according 
to the attraction of the planets, especially of Jupiter. 
In recent times it has been observed somewhere at almost 
every return. Its last return was in September, 1901. 

What has given this comet its celebrity is the theory of 
Encke that its orbit was continually becoming smaller, 
probably through its motion being resisted by eome 
medium surrounding the sun. A number of able mathe- 
maticians have investigated this subject on the various 
returns of the comet. Sometimes there appears to be 
evidence of a retardation, like that found by Encke, and 
sometimes not. The question is, therefore, still in an un- 
settled condition. The computations are so intricate and 
difficult, and, indeed, the whole problem of the motion 
of a comet under the influence of the planets is so compli- 
cated, that it is almost impossible to secure a solution 
which can be guaranteed as absolutely correct. 

Capture of Comets hy Jupiter 

A remarkable case, in w^hich a new comet was made 
a member of the solar system, occurred in the years 1886- 
1889. In the latter year a comet was observed by Brooks 
of Geneva, New York, which proved to be revolving in 



^66 COMETS AND METEORIC BODIES 

an orbit with a period of only seven years. As it was 
quite bright, the question arose why it had never been 
observed before. This question was soon answered by 
the discovery that in the year 1886 the comet had passed 
close to Jupiter. The attraction of the planet had so 
changed its course as to throw the comet into the orbit 
which it now describes. Several other periodic comets 
pass so near to Jupiter that there is little doubt that they 
were brought into the system in this way. 

The question therefore arises whether this may not be 
true of all periodic comets. This question must be an- 
swered in the negative, because Halley'^ comet does not 
pass near any planet. The same is true of Encke's 
comet, which does not come near enough to the orbit of 
Jupiter to have been drawn into its present orbit. With- 
out the action of that planet, so far as we know, these 
comets always have been members of the system. 

Whence Come Comets? 

It was supposed, until a recent time, that comets might 
come into the solar system from the vast spaces between 
the stars. This view, however, seems to be set aside 
by the fact that no comet has been proved to move with 
a much higher speed than it would get by falling to the 
sun from a distance, which, though far outside the solar 
system, is much less than the distance of the stars. We 
shall see hereafter that the sun itself is in motion through 
space. Even if we grant that comets come from space 
far outside the solar sj^stem, the fact that we have just 
cited still "shows that they partook of the motion of the 



WHENCE COME COMETS? 267 

sun and solar system through space wliile tliey were still 
outside that system. 

The view which now seems established by a study of 
the whole subject is that these objects have their regular 
orbits, differing from those of the planets in their great 
eccentricities. Their periods of revolution are generally 
thousands, and sometimes tens of thousands, and even 
hundreds of thousands of years. During this long inter- 
val they fly out to an enormous distance beyond the con- 
fines of the system. If, as they return to the sun, they 
chance to pass very near a planet, two things may hap- 
pen: Either the comet may be given an additional swing 
that will accelerate its speed, throw it out to a greater 
distance than it ever had before or possibly to a distance 
from which it can never return, or the speed may be re- 
tarded and the comet made to move in a smaller orbit. 
Thus it is that we have comets of so many different 
periods. If comets come from the regions of the fixed 
stars, there is no reason why the motion of one might 
not be directly toward the sun, so that it would fall into 
our central luminary. But such an occurrence is hardly 
possible when the comet belongs to our system, because 
one of these bodies nearing an orbit passing through the 
sun would have fallen into the sun on its first round, long 
ages ago, and never could have a chance to fall in again. 

Brilliant Comets of Our Time 

The very bright comets which appear from time to 
time are of the greatest interest to every beholder. It is 
purely a matter of chance, so far as our knowledge ex- 



268 COMETS AND METEORIC BODIES 




Fig. 46. — DonaWs Cornet^ as drawn by G. P. Bond, 



BRILLIANT COMETS OF OUR TIME 269 

tends, when one shall appear. Of what are called great 
comets, there were, five or six during the nineteenth cen- 
tury. The most remarkable and brilliant of all appeared 
in 1858, and bears the name of Donati, its discoverer, an 
astronomer of Florence, Italy. Its history will show the 
changes through which such a body goes. It was first 
seen on June second, but was then only a faint nebulosity, 
visible in the telescope like a minute white cloud in the 
heavens. No tail was then visible, nor was there the 
slightest indication of what the little cloud would grow 
into until the middle of August. Then a small tail 
gradually began to form. Early in September the ob- 
ject became visible to the naked eye. From that time it 
increased at an extraordinary rate, growing larger and 
more conspicuous night after night. Its motions were 
such that it seemed to move but little for the period of a 
whole month, floating in the western sky night after 
night. It attained its greatest brilliancy about October 
tenth. Careful drawings of it were made from time to 
time by George P. Bond, of the Harvard Observatory. 
We give two of these, one a naked eye view, the other a 
telescopic one showing what the head of the comet looked 
like. After October tenth it rapidly faded away. It soon 
travelled toward the south, and passed below our horizon, 
but was followed by observers in the southern hemisphere 
until March, 1859. 

Before the comet had passed out of sight, computers 
began to calculate its orbit. It was soon found not to 
move in an exact parabola, but in a very elongated ellipse. 
The period was not far from nineteen hundred years, but 



270 COMETS AND METEORIC BODIES 




Fig. 47. — Head of DonatVs Cornet^ drawn hy G. P. Bond. 



BRILLIANT COMETS OF OUR TIME 271 

may have been a hundred years more or less than this. It 
must therefore have been visible at its preceding return 
sometime in the first century before Christ, but there is 
no record by which it could be identified. It may be 
expected again in the thirty-eighth or thirty-ninth 
century. 

A very remarkable case of several comets moving in 
very nearly the same orbit is afforded by the comets of 
1843, 1880, and 1882. The first of these was one of the 
most memorable comets on record, as it passed so near 
the sun as almost to graze the surface. In fact, it must 
have passed quite through the outer portions of the 
solar corona. It came into view with remarkable sudden- 
ness in the neighbourhood of the sun, about the end of 
February. It was visible in full daylight. By a singular 
coincidence it appeared shortly after the well-known pre- 
diction of Miller that the end of the world was to come 
in the year 1843. Those who had been alarmed by this 
prediction saw in the comet an omen of the approaching 
catastrophe. 

The comet disappeared from view in April, so that the 
time of observation was rather short. The period of 
revolution now became a subject of interest. It was 
found, however, that its orbit did not differ sensibly from 
the parabola. But the time of observation was so brief 
that any estimate of the period would be somewhat un- 
certain. All that could be said was that the comet would 
not return for several centuries. 

Great, therefore, was the surprise when, thirty-seven 
years later, a comet was seen by observers in the southern 



272 COMETS AND METEORIC BODIES 



Fig. 48.— Great Comet of 1859, drawn by G. P. Bond. 



BRILLIANT COMETS OF OUR TIME 273 

hemisphere and found to be moving in almost the same 
orbit. The first sign which it gave of its approach was 
its long tail rising above the horizon. This was seen in 
the Argentine Republic, at the Cape of Good Hope, and 
in Australia. Not until the fourth of February did the 
head become visible. It swept around the sun, again 
passed to the south, and disappeared without observers 
in the northern hemisphere seeing it. 

The question now arose whether this could possibly be 
the same comet that had appeared in 1843. Previously 
it had been supposed that when two such bodies moved in 
the same orbit with a long interval between they must be 
the same. In the present case, however, the hypothesis of 
identity seemed to be incompatible with the observations. 
The question was set at rest by the appearance in 1882 
of a third comet moving in about the same orbit. This 
certainly could not be a return of the comet which had 
appeared a little more than two years before. The re- 
markable spectacle was therefore offered of three bright 
comets all moving in the same orbit at varying intervals 
of time. Possibly there were more even than these three, 
for, in 1680, a comet had passed very near the sun. Its 
orbit, however, was somewhat different from those of the 
three comets already mentioned. 

The most probable explanation of the case seems to be 
that these comets were parts of some nebulous mass which 
gradually broke up, its different members pursuing their 
courses independently. The result would be that, for 
many ages, the objects would all continue in nearly the 
same orbit. 



274 COMETS AND METEORIC BODIES 

Besides these, brilliant comets appeared in 1859, 1860, 
and 1881. How long we may have to wait for another 
no one can say. It is probable that Halley's comet, when 
it appears eight or ten years hence, will at least be visible 
to the naked eye, but no one can predict even its apparent 
brightness. At its return in 1835 it was so small an 
affair that it was difficult to explain the excitement it 
caused in 1456 and later, except by supposing a great 
diminution in the dimensions, at least of its tail. 

Nature of Comets 

The question of the exact nature of a comet is still in 
doubt. In the case of large and bright comets, it is possi- 
ble that the nucleus may be a solid body, though probably 
much smaller than it looks. Some light on the question 
is thrown by an observation, which is unique, made at the 
Cape of Good Hope when the great comet of 1882 
made a transit across the sun's disk, as Mercury and 
Venus are sometimes known to do. Unfortunately, as- 
tronomers generally were not prepared for such a phe- 
nomenon, as the comet had been visible only in the 
southern hemisphere, and the transit occurred only a 
week or two after its first discovery. Hence it happened 
that the Cape Observatory was the only one at which an 
observation of the greatest interest in astronomy could 
be made ; and here the circumstances were extremely un- 
favourable. The sun was about to set behind Table Moun- 
tain as the comet approached it. By careful watching, 
two of the astronomers, Messrs. Finlay and Elkin, were 
enabled to keep sight of the comet until it actually disap- 



NATURE OF COMETS 275 

peared at the limb of the sun. This happened fifteen 
minutes before the sun disappeared from view. During 
this time, if the nucleus were a solid body, it ought to 
have been seen as a black spot projected against the sun. 
Nothing of the sort could be made out. The conclusion 
is either that the substance of the comet was transparent 
to the sun's rays, or that the solid nucleus was too small to 
be distinguished under the circumstances. Unfortunate- 
ly, owing to the low altitude of the sun and the bad condi- 
tion of the air, it was impossible to be quite sure how 
small the nucleus must have been to be invisible. It 
seemed certain, however, that the solid portion, if any 
such the comet had, w^as much smaller than the apparent 
nucleus as seen in the telescope. 

There seems also to be some reason for suspecting that 
a comet is nothing but a collection of meteoric matter, 
consisting perhaps of separate objects, of sizes ranging 
anywhere from that of grains of sand to masses as large 
as the meteorites which sometimes fall from the sky. The 
question then is to explain how the parts are kept to- 
gether through so many revolutions of the comet. The 
changes of shape which the nucleus often undergoes as it 
is passing near to the sun seem to show that this hypothe- 
sis may be near the truth. 

The spectra of those comets whose light has been 
analysed by the spectroscope are remarkable in showing 
that this light is not merely reflected sunlight. The 
principal feature is three bright bands, which bear a 
striking resemblance to those given by the compounds 
of carbon and hydrogen. Taking this fact by itself, the " 



276 COMETS AND METEORIC BODIES 

conclusion would be that the comet is a glowing gas, 
shining as incandescent gases do in our chemical labora- 
tories. That such should be the case and the whole case 
seems impossible for two reasons. The comet cannot be 
hot enough to glow; and its light fades out to nothing 
as it recedes from the sun. The most likely conclusion 
seems to be that the action of the sun's rays causes a glow 
through some process which has not yet been made clear 
to us. 

What seems certain is that the matter of which a bright 
comet is composed is volatile. When a bright comet is 
carefully scrutinised with a telescope, masses of vapour 
can be seen from time to time slowly rising from its head 
in the direction of the sun, then spreading out and mov- 
ing away from the sun so as to form the tail. The latter 
is not an appendage which the comet carries as animals 
carry their tails, but is like a stream of smoke issuing 
from a chimney. 

It frequently happens that when a comet is first dis- 
covered it has no tail at all. The latter begins to form 
when the sun is approached. The nearer the comet ap- 
proaches the sun, and the greater the heat to which it is 
exposed, the more rapidly the tail develops. All this 
shows that the matter which composes a great comet is, 
in part volatile. When warmed by the heat of the sun it 
begins to evaporate, just as water would under the same 
circumstances. The steam or vapour thus arising is re- 
pelled by the sun, so as to form a stream of matter issu- 
ing from the comet. 



II 

Meteoric Bodies 

Every reader of this book must frequently have seen 
what is familiarly called a "shooting star" — an object 
like a star, which darts through the heavens a greater or 
less distance, and then disappears. These objects are, in 
astronomy, called by the generic name of meteors. They 
are of every degree of brightness, but the brighter they 
may be, the more rarely they appear. One who is out 
much at night will seldom pass a year without seeing such 
a meteor of striking brilliancy. Once or twice in a life- 
time he will see one that illuminates the whole sky with 
its light. 

On almost any clear night in the year a watcher may 
see three or four or even more meteors in the course of an 
hour. Sometimes, however, they are vastly more numer- 
ous, for example, between the tenth and fifteenth of 
August, more and brighter ones than usual will be seen. 
On a number of occasions in history they have coursed the 
heavens in such numbers as to fill the beholders with sur- 
prise and terror. There were remarkable cases of this 
kind in 1799 and 1833. In the latter year, especially, 
the negroes of the South were so terrified that the recol- 
lection of the phenomenon is brought down by tradition 
to the present day. 



278 COMETS AND METEORIC BODIES 

Cause of Meteors 

The cause of meteors was unknown until after the be- 
ginning of the nineteenth century. It is now, however, 
well made out. Besides the known objects of the solar 
system — planets, satellites, and comets — there are, cours- 
ing through space, and revolving around the sun, count- 
less millions of particles, or minute collections of matter, 
too small to be seen with the most powerful telescope. 
Quite likely the greater number of these objects are 
scarcely larger than pebbles, or even grains of sand. The 
earth, in its course around the sun, is continually encoun- 
tering them. One in the line of motion of the earth 
may have a velocity amounting to many miles a sec- 
ond; perhaps ten, twenty, thirty, or even forty. Meet- 
ing the atmosphere with this immense velocity causes the 
body to be immediately heated to so high a temperature 
that its substance dissolves away with a brilliant effusion 
of light no matter how solid it may be. What we see 
is the course of a particle thus burning away as it darts 
through the rare regions of the upper atmosphere. 

Of course, a meteor will appear brighter and last 
longer the larger and solider it is. Sometimes it is so 
large and solid that it comes within a few miles of the 
earth before being finally melted and dissolved away. 
Then, the people in the region over which it is passing, 
see a remarkably bright meteor. In such a case it fre- 
quently happens that in a few minutes after the meteor 
has passed a loud explosion, like the firing of a cannon, 
is heard coming from the region through w^hich it passed. 



METEORIC SHOWERS 279 

This arises from the concussion of the air compressed by 
the rapid flight. 

In rare cases the mass is so large that it reaches the 
earth without being melted or evaporated. Then we have 
the fall of a meteoric stone, as it is called, which com- 
monly occurs several times a year in some part or another 
of the world. There is at least one case on record in 
which a man was killed by the fall of such a body. When 
these stones are dug up they are found to be composed 
mostly of iron. Specimens of them are kept in our 
museums, where they may be examined by anyone who 
wishes to see them. Some remarkable ones are found at 
the Smithsonian Institution, Washington, D. C. 

How these objects originated we cannot say, and even 
a guess on the subject would be hazardous. When found 
they bear marks on their surface of having been melted ; 
this, however, is a natural result of their passage through 
the air, by which the surface is always heated far above 
the melting point. 

Meteoric Showers 

The greatest discovery of our times on the subject of 
meteors is connected with the meteoric showers already 
referred to, which occur at certain seasons of the year. 
The most remarkable of these occur in November, and the 
meteors of the shower are called Leonides, because their 
lines of apparent motion all diverge from the constella- 
tion Leo. It was found by historical research on the 
subject that this shower had recurred at intervals of 
about one third of a century for at least thirteen hundred 



280 COMETS AND METEORIC BODIES 

3^ears. The earliest account is the following from an 
Arabian writer : 

In the year 599, on the last day of Moharren, stars shot hither 
and thither, and flew against each other like a swarm of locusts ; 
people were thrown into consternation and made supplication to 
the Most High; there was never the like seen except on the 
coming of the messenger of God ; on whom be benediction and 
peace. 

The first well-described shower of this class occurred on 
November 12, 1799. It was seen by Humboldt, then on 
the Andes. He seems to have considered it as a very re- 
markable display, but made no exact investigation as to 
its cause. 

The next recurrence was in 1833, which seems to have 
been the most remarkable one ever observed. The as- 
tronomer Olbers suggested from this that the shower had 
a period of thirty-four years, and predicted a possible 
return in 1867, which actually appeared in 1866. In 
1866 and 1867 the observations were more carefully 
made than ever before, and led to the remarkable astro- 
nomical discovery, just alluded to, that of the relation 
between meteors and comets. To explain this we must 
define the radiant point of meteors. 

It is found that if, during a meteoric shower, we mark 
the course of each meteor by a line on the celestial sphere, 
and continue these lines backward, we shall find them all 
to meet at a certain point in the heavens. In the case 
of the November meteors this point is in the constellation 
Leo ; in the August meteors it is in Perseus. It is called 
the radiant point of the shower. The lines in which the 



COMETS AND METEORS 281 

meteors move are the same as if they were all shot out 
from this one point, but it must not be supposed that the 
meteors are actually seen at this point; they may begin 
to show themselves at any distance from it less than 
ninety degrees ; but when they are seen they are moving 
from the point. This shows that the meteors are all mov- 
ing in parallel lines when they encounter our atmosphere. 
The radiant point is what, in perspective, is called the 
vanishing point. 

Connection of Comets and Meteors 

The period of the November meteors, thirty-three 
years, being known, and the exact position of the radiant 
point determined, it became possible to calculate the orbit 
of these objects. This was done by Leverrier soon after 
the shower of 1866. Now it happened that, in December, 
1865, a comet appeared which passed its perihelion in 
January, 1866. Careful study of its motion showed that 
its period was about thirty-three years. This orbit was 
computed by Oppolzer, who published it without noticing 
its resemblance to that of the meteors. Then it was no- 
ticed by Schiaparelli that there was an almost perfect re- 
semblance between the orbit of Oppolzer's comet and the 
Leverrier orbit of the November meteors. So near to- 
gether were they that no doubt could be felt that the two 
orbits were identical. The evident fact was that the 
bodies which produced these November meteors were fol- 
lowing the comet in its orbit. It was therefore concluded 
that these objects had originally formed part of the 
comet and had graduall}^ separated from it. When a 



282 COMETS AND METEORIC BODIES 

comet is disintegrated in the manner described in the last 
chapter, those portions of its mass which are not com- 
pletely dissipated continue to revolve around the sun as 
minute particles, which get gradually separated from 
each other in consequence of there being no sufficient bond 
of attraction, but they still follow each other in line in 
nearly the same orbit. 

The same thing was found to be true of the August 
meteors. They are found to move in an orbit very near 
to that of a comet observed in 1862. The period of this 
comet could not be exactly determined, but it is supposed 
to be between one and two hundred years. 

The third remarkable case of this kind occurred in 
1872. We have already spoken of the disappearance of 
Biela's comet. It happens that the orbit of this body 
nearly intersected that of the earth at the point which 
the latter passes toward the end of November. From the 
observed period of this comet it should have passed this 
point about the first of September, 1872, between two 
and three months before the passage of the earth 
through the same point. From the analogy of the other 
cases it was therefore judged that there would be a 
meteoric shower on the evening of November 27, 1872, 
and that the radiant point would be in the constellation 
Andromeda. This prediction was fulfilled in every re- 
spect. The Andromedes, as these meteors are called, now 
recur with great regularity. 

There are now some disappointing circumstances to 
narrate. The comet of 1866 should have reappeared 
sometime during the years 1898-1900, but it was not 



COMETS AND METEORS 283 

seen. Probably it was missed, not because of its com- 
plete disintegration, but because it happened to pass its 
perihelion at a time when the earth was too far away to 
admit of the comet being visible. But, what is still more 
curious is that the meteors themselves, a shower of which 
was expected in 1899-1900, did not reappear in great 
numbers at either date. The probable reason for this is 
that the swarm was deflected from its course by the at- 
traction of the planets, w^hich continually changes the 
orbit of every object of this kind. 

The general conclusion is that the countless thousands 
of comets which in time past have coursed around the 
sun, leave behind minute fragments of their mass, which 
follow in their orbits like stragglers from an army, and 
that, when the earth encounters a swarm of these frag- 
ments a meteoric shower is produced. But it is still an 
open question whether all these meteoric particles can 
be fragments of comets, with the probabilities in favor 
of a negative answer. If we are to accept the conclu- 
sions drawn by Professor Elkin from recent photographs 
of meteors, the velocities of these bodies sometimes exceed 
the parabolic limit described in the last chapter. If this 
be so, they must be wanderers through the infinite stellar 
spaces, having no connection with our system. 

The Zodiacal Light 

This is a very soft, faint light, surrounding the sun, 
extending out to about the orbit of the earth, and lying 
nearly in the plane of the ecliptic. In tropical latitudes 
it may be seen on any clear evening about an hour or 



284 COMETS AND METEORIC BODIES 

less after sunset. In our latitudes it is best seen in 
the spring, when, about an hour and a half after sunset, 
it may always be seen in the west and southwest, extend- 
ing upward toward the Pleiades. It is best seen at this 




Fig. 49. — The Zodiacal Light in February and March. 

season because, lying in the plane of the ecliptic, it makes 
a greater angle with the horizon then than at other sea- 
sons. In autumn it may be seen in the morning before 
daybreak, rising from the east and extending toward 
the south. 

It is said that in regions where the atmosphere is 
clearer than with us, it may be seen all night, spanning 
the heavens like a complete circle. If so, the light is so 



THE ZODIACAL LIGHT 285 

faint as to elude ordinary vision, and this continuity does 
not seem to be well established. 

But there is associated with it a phenomena which is 
still one of the mysteries of astronomy. In the heavens, 
immediately opposite the sun, there is always a faint 
light, to which the term Gegenschein is applied. This is 
a German word, of which the best English equivalent is 
counter- glow. The light is so faint that it can be seen 
only under the most favourable conditions. When it falls 
in the Milky Way the light of that body is sufficient to 
drown it out, as is that of the moon, if the latter is above 
the horizon. 

It passes through the Milky Way in June and Decem- 
ber of each year, and can therefore not be seen during 
these months. Nor is it likely to be seen during the first 
part of January or July. At other times it must be 
looked for when the sun is considerably below the horizon, 
the sky perfectly clear and the moon not in sight. It 
may then be seen as an extremely faint impression of 
light, to which no exact outline can be assigned. The 
observer will find it by sweeping his eye over the region 
of the spot exactly opposite the sun. 

There can be little doubt that the zodiacal light is 
caused by the reflection of the light of the sun from a 
swarm of very minute bodies, perhaps in the nature of 
meteors, continually revolving around it. We might 
naturally attribute the Gegenschein to the same cause, 
but the question would then arise why it is only seen 
opposite the sun. It has been suggested that possibly 
the earth has a tail, like a comet, and that the Gegen- 



286 COMETS AND METEORIC BODIES 

schein is simply this tail seen endwise. This is not an 
impossibility, but there is no proof that it is true. 

The Impulsion of Light 

Facts are now being discovered, and physical theories 
developed, the ultimate outcome of which may be an ex- 
planation of a number of mysterious phenomena asso- 
ciated with the earth and the universe. These phenomena 
are presented by the corona of the sun, the tails of comets, 
the aurora, terrestrial magnetism and its variations, 
nebulae, the Gegenschein, and the zodiacal light. The 
theories in question belong rather to the physicist than 
the astronomer, and the writer does not feel competent to 
explain them fully in their latest form, nor to define 
where established facts end and speculation begins. He 
must therefore limit himself to a few points. 

First in order we have a pressure exerted by light, 
which was pointed out by Maxwell thirty years ago, but 
which seems to have been very generally overlooked, by 
astronomers at least. This principle was deduced by 
Maxwell from the electro-magnetic theory of light, and 
may be stated as follows: 

When a pencil of light impinges perpendicularly on 
an opaque object, it produces a pressure upon the surface 
of that object, determined by the condition that if the 
object were set in motion with the velocity of light, and 
the force against it were kept up, the power required to 
keep up the pressure would be equal to that carried by 
the ray of light. 

Another way of expressing the principle is this : Sup- 



THE IMPULSION OF LIGHT 287 

posing the rays of light to be parallel, the work done by 
the pressure upon a surface moving through any length 
of the pencil is equal to the energy of the light con- 
tained in that length. 

By the aid of this principle and a knowledge of the 
heat or energy contained in the rays of the sun, it is 
possible to calculate the pressure in question. It is found 
to be too slight to be detected by any ordinary mode of 
measurement. The great difficulty arises from the fact 
that, if the experiment is not tried in a vacuum, the pres- 
sure will be confused with that exerted by the surround- 
ing air. A vacuum so nearly perfect that the slight 
residuum of air still contained within it shall not exert a 
force comparable with the light has not yet been at- 
tained. Our conclusion must therefore depend on obser- 
vations made on minute particles contained in the celes- 
tial spaces; and we cannot ascend into these spaces to 
make the experiments, nor can we send matter up there 
to be experimented upon. All we can do is to observe 
matter already at hand. Here, then, is a wide gap which 
we cannot bridge over in practice. 

The other element in the case is the discovery that par- 
ticles smaller than atoms, called corpuscles or ions^ are 
thrown off with high velocity from intensely heated 
bodies. The sun being such a body, it follows that such 
ions must be shot out from it. 

On Maxwell's theory, the explanation of a comet's tail 
is simple in the extreme. Being in the vacuum of celes- 
tial space, the matter of the comet evaporates on the 
side next to the sun, and, there being no pressure to hin- 



288 COMETS AND METEORIC BODIES 

der its expansion, it begins b}^ flying off in all directions, 
especially toward the sun. It condenses into very minute 
particles, which are acted upon by the sun's rays and 
thus tlirown in the direction away from the sun. That 
the tail of the comet was produced by a repulsion like this 
has been evident ever since observations were made, but 
not until Maxwell's law was understood could any ex- 
planation be given of the seeming repulsion of the matter 
of the tail by the sun. 

The explanations of the other phenomena we have men- 
tioned are not yet so simple and satisfactory that they 
may be clearly stated in a short space. The reader who 
is interested in the subject must therefore be referred to 
special papers and treatises.* 

♦The papers to whicli the present writer is principally indebted for the 
views in question are hy Prof. J. J. Thompson, in the Popular Science Monthly 
for August, 1901, and to the article by Prof, John Cox in the number for Jan- 
uary, 1902. These papers again set forth the investigations of Arrhenius, the 
Swedish physicist, who seem^ to have made the most successful endeavoiir 
to explain the phenomena in question on the principles which we have 
mentioned. 



PART VI 
THE FIXED STARS 



I 

Generai. Review 

Having completed our survey of that small section of 
the universe in which we have our dwelling, our next task 
is to fly in imagination to those distant parts of space 
occupied by the thousands of stars which stud our sky. 
This is the field of astronomy in which the most wonder- 
ful discoveries have been made in recent times. We now 
know things about many stars which, even to such an 
observer as Sir William Herschel, would have seemed far 
beyond the possibilities of human ken. But the very 
vastness of the field and the minuteness of the details 
into which recent research has gone render it impossible 
to undertake anything like a comprehensive survey within 
the limits of the present little book. All we can do is to 
point out the more salient features of the universe of stars 
as they have been brought to light by observers and in- 
vestigators of the past and present. The reader who 
desires further details and a wider idea of the methods 
and results of recent research relating to the stars may 
find them in a volume which the present author has re- 
cently devoted to the subject.*" 

From the childhood of the race men have inquired: 
"What is a star .f^" To this question no answer was pos- 



*The Stars, a Study of the Universe. G. P. Putnam's Sons, New York. 



292 THE FIXED STARS 

sible until recent times. Even within the last century 
little more could be said than that they were shining 
bodies whose nature was to us a mystery. At the present 
time we may define the stars as immense globes of matter, 
generally millions of times the size of the earth, so in- 
tensely hot that they shine by their own light, and so 
massive that they may continue to give light and heat for 
unknown millions of years without cooling off. What 
we have said of the sun probably applies in a greater or 
less degree to the great majority of the stars. It is true 
that we cannot study their surfaces because, even in the 
most powerful telescopes, they appear as mere points of 
light. But the analogy with our sun and with other 
heavenly bodies leads us to believe that each of them re- 
volves on its axis as the sun does, and that, could we see 
it at the proper distance, it would present much the same 
appearance as our sun. We have abundant evidence that 
rotation is the order of nature in the case of all the heav- 
enly bodies. In the few cases where it is possible to 
decide whether a star does or does not rotate, the question 
has been answered in the affirmative. 

There are innumerable differences of detail among the 
stars. Indeed it would seem that no two are exactly alike 
in their physical constitution, any more than two men 
are alike in their personal appearance and make-up. In 
the chapter on the sun we tried to give an idea of the 
enormous temperature of that body, which far exceeds 
any degree of heat we can produce on the earth. We 
have good reason to believe that, while the stars differ 
widely in temperature, the great majority of them are 



STARS AND NEBULiE 293 

far hotter even than the sun. This is true of their sur- 
faces and must be still more true of their vast interiors. 

Stars and Nehulm 

Stars are not the only bodies which fill these distant 
regions of space. Scattered over the sky are immense 
masses of exceedingly rare matter which, from their 
cloud-like appearance, are called nehulce. In size these 
bodies far exceed the sun or stars. A nebula only as 
large as our solar system would probably be invisible in 
the most powerful telescope, and could never be impressed 
even on the most delicate photograph of the sky unless 
above the ordinary brightness. Those that we know have 
probably hundreds or thousands of times the extent of 
our whole solar system. We may therefore classify those 
bodies of the universe which shine by their own light as 
stars and nebulas. 

Spectra of the Stars 

When we read of astronomical discoveries, we common- 
ly think of them as being made by looking through a 
telescope. But this is no longer the case. The greatest 
astronomical development of recent times consists in 
proving the existence of dark bodies of the nature of 
planets, revolving around many stars. These objects 
are absolutely invisible in any telescope which it would 
be possible to construct. Such an instrument could tell 
us nothing about the constitution of a star. The great 
engine of progress has been the spectroscope, which is 
described in a previous chapter. From what has there 



294 THE FIXED STARS 

been said the reader will see that, using words in their 
ordinary sense, we do not see anytliing by the aid of a 
spectroscope. What we do with it is to analyse the rays 
of light into their component parts, just as a chemist 
analyses a compound body into its simple elements. A 
spectroscopic analysis is more complicated from the fact 
that the number of elements which compose a ray of light 
is generally indefinite. The great advantage of spectro- 
scopic analysis arises from the fact that it is independent 
of distance. The farther a star is away, the more diffi- 
cult it is to see, whether we look at it with the naked eye 
or through a telescope. Its light diminishes as the square 
of the distance increases ; twice as far away it gives us 
only one fourth the light ; three times as far away, only 
one ninth the light, and so on. But if enough light comes 
from the star to enable its spectiTim to be analysed, the 
result can be reached equally well no matter how great 
the distance. As the chemist could analyse a mineral 
brought from the planet Mars, were such a thing pos- 
sible, as easily as he could if he found it on the earth, 
so, when a ray of light reaches the spectroscope, the fact 
that it may have been hundreds of years on its way, does 
not interfere with the drawing of conclusions from it. 

When the spectrum of a star is formed it is always 
found to be crossed by numerous dark lines. This shows 
that all the stars, like our sun, are surrounded by atmos- 
pheres which are not as hot as the central body. But this 
does not imply that the atmosphere is cold. On the con- 
trary, it is probably hotter than the flame of any furnace 
we have on earth, even in the case of the cooler stars. 



SPECTRA OF THE STARS 295 

When the spectra of stars are carefully compared, it is 
always found that hardly any two are exactly alike. 
This shows that their atmospheres all differ in their phys- 
ical constitution, or in the temperature of the substances 
which compose them. A great number of the dark lines 
of their spectra are found to be identical with those pro- 
duced by known substances on earth. This shows that 
the substances of which the stars are made up are iden- 
tical, in at least a great part, with those on the earth. 

One of the most abundant of these substances is hydro- 
gen. Several lines of hydrogen are found in nearly all 
the stars. Another substance which seems to be almost 
universal throughout the universe is iron. Yet another is 
calcium, the metallic base of lime. We all know that this 
substance abounds on the earth, and we have, in its diffu- 
sion among the stars, an example of the unity of nature 
in its widest extent. ^ 

Yet, variety is also the rule. Besides lines due to known 
substances, many stars show lines which have not yet been 
identified with those of any element that we know of. 
This is especially the case in the class known as Orion 
stars, because many of them are found in the constella- 
tion Orion. These stars are mostly very white or even 
blue in colour, and show a number of fine dark lines which 
are to a greater or less extent the same in all Orion stars, 
but are not those produced by any known chemical 
element. We therefore have reason to believe that there 
are in the stars other chemical elements than those with 
which we are acquainted. 

There is a very curious case in which an element first 



296 THE FIXED STARS 

excited interest through its being found in the sun and 
stars. For some time after the study of the sun's spec- 
trum had been commenced, it was known that certain well- 
marked lines in it were not produced by any substance 
then known. But continued research led to the discovery 
that this substance existed in a Norwegian mineral, 
cleveite, and perhaps elsewhere on the earth. From its 
existence on the sun it was called helium. Its spectrum 
was no sooner made known than it was found that helium 
existed in many stars which are, for that reason, called 
"helium stars." 

Density and Heat of the Stars 

In many cases some idea can be obtained of the 
density of a star, or, in ordinary language, of its 
specific gravity. It is very remarkable that, in near- 
ly all such cases the density is found to be far less 
than that of our ordinary solid or liquid substances ; 
frequently no greater than that of air, sometimes 
even less. In this respect our sun, although its den- 
sity is so small, seems to be an exception, and it is 
likely that only a very small proportion of the stars are 
as dense as the sun. This affords one proof of the high 
temperature of these bodies, which must be such that all 
liquid or solid substances exposed to it would boil away 
as water boils when put on a fire, thus changing 
its substance into a vapour. We have reason to be- 
lieve that the stars are for the most part masses of 
this intensely hot vapour, surrounded perhaps by a 
somewhat colder surface. Possibly many of the stars 



DENSITY AND HEAT OF THE STARS 297 

may be of the nature of bubbles, but this is far from 
being estabhshed. 

A star, Hke the sun, must be hotter in the interior than 
at its surface. From the latter alone can heat be radiated ; 
hence the surface is continually cooling off, and if the 
matter composing the body were at rest, the cooling 
would soon go so far that a crust would form, as it does 
on a mass of molten iron. The only way in which this 
can be prevented is that, as the superficial portions cool, 
the greater density which they thus acquire causes them 
to sink down into the seething mass below, portions of 
w^hich arise to take their place, cool off, and sink in their 
turn. Thus there is a continual interchange of matter 
between the inside and the surface, much as in a boiling 
pot the water at the bottom is continually being forced 
up to the top, while that on the top continually sinks 
down. 

It follows from this that there must be a limit to the 
smallness of a star. If such a body were no larger than 
the moon, it would, in a few thousand years, so far cool 
off that a crust would form over its surface. This would 
cut off the currents by which the hot matter is brought 
to the surface and the star would soon cease to shine. As 
there can be little doubt that the age of most of the stars 
is to be reckoned by millions of years, it follows that they 
must be so large that they can lose heat for millions of 
years and yet a cool crust not form on their surface. 

We have said that our sun is among the colder of the 
stars and also that it is among the smaller. These two 
facts fit well together. The smaller a star is the more 



298 THE FIXED STARS 

rapidly it cools off, just as a cup of water cools off faster 
than a pot full. 

The revelations of the spectroscope makes it very prob- 
able that every star has a life history. It began as a 
nebula, which, in the course of ages, slowly condensed 
into an intensely hot, blue-coloured star. The condensa- 
tion going on, the star becomes yet hotter, until it reaches 
its highest temperature. Then, cooling off, its colour 
changes to white, yellow, and red, and the lines in its 
spectrum become darker and more numerous. Finally, its 
light dies away, as a fire flickers out when the supply of 
fuel is exhausted, and the star becomes a dark opaque 
body, — its life has ended. The greater the mass of the 
star the longer its life. Thus it is that the stars we 
observe seem to be of all ages, from the infantile nebula 
to the star dying of old age. 



II 

Aspect of the Sky 

Not only to the ordinary beholder, but to the learned 
student of the heavens, the most wonderful feature of 
the sky is the Milky Way. This is a girdle apparently 
spanning the sky and perhaps, in reality, spanning the 
entire universe of stars, uniting them, as it were, into a 
single system — one "stupendous whole." It may be seen 
at some time of the night every day of the year, and at 
some convenient hour in the evening of every month ex- 
cept May. During this month it extends round the 
horizon in the early evening, and is invisible through 
the denser strata of the air. Of course it will even 
then become visible in the east and northeast later 
at night. 

The smallest telescope will show the Milky Way to be 
formed of immense congeries of stars, too faint in their 
light to be separately visible at their great distance from 
us. Careful observation, even with the naked eye, will 
show that these stars are not equally scattered along the 
whole extent of their course, but are frequently collected 
in great masses or clusters, with comparatively empty 
spaces around or between them. These are especially 
marked in the portions of the belt visible in the south in 
the evenings of summer and autumn. 

A remarkable fact connected with the universe is that 



300 THE FIXED STARS 

the stars are not equally thick in all directions, there be- 
ing more in a given space around the belt of the Milky 
Way, and the number growing smaller as we pass away 
from that belt. This is true even of the brightest stars, 
and yet more true of the fainter ones. The poles of the 
Milky Way are those two points in the heavens which 
are ninety degrees from every point of the Milky Way. 
If we imagine one to hold a rod in his hand, so that the 
Milky Way shall be at right angles to it, the two ends of 
the rod w^ill point to the two poles in question. To give 
an idea of the thickness of the stars w^e may say that, 
near the poles of the Milky Way, a round circle of the 
sky one degree in diameter will commonly contain two 
or three stars visible in quite a small sized telescope. In 
the region of the Milky Way, such a circle may contain 
eight, ten, perhaps even fifteen or twenty such stars. 

Brightness of the Stars 

No one can look at the sky without seeing that the 
stars differ enormously in their brightness, or, in the 
language of astronomy, in their magnitude. They re- 
semble men in that a very few far outshine all their fel- 
lows, a greater number are less bright, and, as we come 
down to smaller and smaller stars, we find the number 
to continually increase. Those visible to the naked eye 
were classified by the ancient astronomers as of six orders 
of magnitude. About twenty of the brightest in the sky 
were designated as of the first magnitude. The forty 
next in order of brightness were called of the second mag- 
nitude ; a larger number were of the third, and so on to 



BRIGHTNESS OF THE STARS 301 

the sixth magnitude, which included the faintest stars 
that the best eye could see under a clear sky. 

Modern astronomers carry this system down to the 
telescopic stars. Those which are one degree fainter than 
the smallest visible to the naked eye are called of the 
seventh magnitude; the next in brightness are of the 
eighth, and so on. The faintest that can be seen or 
photographed with the largest telescopes are probably 
of the fifteenth, sixteenth, or seventeenth magnitude. 

The reader will of course understand that the magni- 
tude of a star does not express its real brightness, because 
a shining body looks brighter the nearer it is to us. No 
matter how bright a star may be, if it were removed far 
enough away it would grow so faint as to be invisible. 
The smallest star in the heavens if brought near enough 
to us would shine as of the first magnitude. 

It was formerly believed that the actual brightness of 
the different stars was nearly the same, and that some 
looked brighter than others only because they were 
nearer to us. But the case is now known to be different. 
Estimates of the distance of the stars show that among 
the nearest to us are many quite invisible to the naked 
eye, while some of the first magnitude are so far away 
that their distance is immeasurable. The brightest ones 
probably emit hundreds of thousands of times as much 
light as the smallest ones. 

Number of Stars 

The whole number of stars in the heavens which can 
be seen by the ordinary eye is between five and six thou- 



302 THE FIXED STARS 

sand. Possibly a very keen eye might see more than six 
thousand, but most eyes will see even less than five thou- 
sand. Of these only one half can be above the horizon at 
the same time, and of this half a great number will be 
so near the horizon as to be obscured by the great thick- 
ness of the atmosphere in that direction. The number 
which can readily be seen on a clear evening by an 
ordinarily good eye will probably range between fifteen 
hundred and two thousand. Stars visible to the naked 
eye are called lucid stars, to distinguish them from tele- 
scopic stars, which can be seen only by the aid of a 
telescope. 

It is impossible to make even an estimate of the total 
number of telescopic stars. It is commonly supposed 
that between fifty and one hundred million can be seen 
with large telescopes, and it is now possible, with spe- 
cially arranged telescopes, to photograph stars which 
are fainter than the smallest the eye can see in any tele- 
scope. There is no sign of any limit to the number. As 
we pass to fainter and fainter degrees of brightness the 
stars are found to be more and more numerous. All that 
we can say of the total number is that it must be counted 
by hundreds of millions. 

We have, in fact, some reason for inferring that the 
great majority of the stars are invisible in the most 
powerful telescope we can make, owing to their distance. 
The distance of the great majority is such that only the 
brightest of them can become known to us. 

Minute stars are here and there collected into clusters 
in various parts of the sky. Some of these clusters are 



NUMBER OF STARS 303 

visible to the naked eye. Those in and near the Milky 
Way frequently contain hundreds or even thousands of 
stars too small to be seen separately without a telescope. 
The stars differ from each other in colour, although 
not in so marked a degree as terrestrial objects. The 
most casual observer cannot fail to note the difference 
between the bluish white of Alpha Lyrae and the reddish 
light of Arcturus. There seems to be a regular grada- 
tion in the colour of the stars from blue, through yellow, 
to red. These differences of colour are connected with 
differences in the spectra of the stars. As a general rule, 
the redder a star is, the greater the number and intensity 
of the dark lines that can be seen in the green and blue 
parts of its spectrum. 

Constellations 

A slight examination of the heavens shows that the 
stars are not scattered equally over the sky, but that there 
is more or less of a tendency to collect into constellations. 
This is especially the case with the brighter stars. But 
no well-marked dividing line between the constellations is 
possible; that is, we cannot draw a line showing exactly 
where one constellation ends and another begins. Never- 
theless a division into constellations was made in ancient 
times and has been followed by astronomers down to the 
present time. 

How and by whom the constellations were first mapped 
out and named no one knows. The Chinese had their 
asterisms — collections of stars smaller than what we call 
constellations — in the earliest years of their history. 



304 THE FIXED STARS 

What we know of the constellations dates from Ptolemy,, 
who lived in the second century after Christ. His names 
are still in use. As many of them are those of the gods, 
goddesses, and heroes of Grecian mythology — Perseus, 
Andromeda, Cepheus, Hercules, etc. — it seems likely that 
they were assigned during or after the heroic age. 

In modern times quite a number of new constellations 
have been carved out of or drawn between the older ones. 
This is especially the case in the southern hemisphere, 
which was imperfectly known to the ancient Greeks. 



Ill 

Description of the Constellations 

The present chapter is intended for those who wish to 
be able to recognise the principal constellations, and to 
know where to look for the several planets. The problem 
of pointing out the constellations is complicated by the 
effect of the twofold motion of the earth ; on its axis and 
around the sun. In consequence of the former the con- 
stellations change their apparent position in the course of 
the night, and the result of the latter is that different 
constellations are seen at different seasons. 

We explained in a former chapter how, in consequence 
of the motion of the earth in its orbit round the sun, the 
latter seems to us to perform an annual circuit among 
the constellations. Hence, if a star is east of the sun, we 
shall see it approach nearer to the sun every day. If we 
look out night after night at the same hour we shall find 
it farther and farther advanced toward the west. In 
consequence of this change it must rise and set earlier 
every day than it did the day before. More exactly, the 
time between two risings and settings of the same star is 
twenty-three hours fifty-six minutes four and a half sec- 
onds. While in the course of a year the sun rises three 
hundred and sixty-five times, a star rises three hundred 
and sixty-six times. The latter will therefore during the 
year have risen at every hour of the day and night. 



306 THE FIXED STARS 

Astronomers avoid all confusion from this cause by 
the use of sidereal time, that is star-time, or time meas- 
ured by the stars. As already explained, a sidereal day 
is the interval between two successive pasages of a star 
over the meridian, and is three minutes fifty-six seconds 
less than our ordinary day. It is divided into twenty- 
four sidereal hours, and each hour into sidereal minutes 
and seconds. A sidereal clock gains three minutes fifty- 
six seconds daily on an ordinary clock and thus shows 
the same time at the same position of the stars the year 
around. 

One who wishes to keep the run of the stars will find it 
very convenient to have some idea of sidereal time. This 
may be had by the following rule : Double the number of 
the month; the product will be the sidereal time at six 
o'clock in the evening. At seven o'clock it will be one 
hour later, and at eight it will be two hours later, and so on. 

Suppose, for example, that one looks at the sky in 
November at nine o'clock in the evening. This is the 
eleventh month; multiplying by two gives twenty-two, 
adding three gives twenty-five, from which we drop 
twenty-four, giving one hour as the sidereal time. The 
time thus obtained will not often be more than an hour 
in error, except during the first week or ten days of the 
month, when it may be an hour or more too great. It 
may then be diminished by one hour. 

Applying the same rule in January we have five hours 
as the sidereal time at nine in the evening. But early in 
the month the sidereal time at nine in the evening will be 
four hours instead of five. 



THE NORTHERN CONSTELLATIONS 307 



At hours sidereal time the equinoctial colure is on 
the meridian ; at six hours, the solstitial colure, and so on. 

The Northern Constellations 

With this preliminary explanation let us proceed to the 
study of the constellations. I assume the reader to be 
somewhere in the latitude of the United States. Then 
the principal northern constellations will never set, and 
will be visible in whole or in part every evening in the 
year. With them, therefore, we begin. 

A figure, showing these constellations, is found in the 
first part of the present book (Fig. 2). To see how they 
will appear hold the cut with the month at top ; we then 
have the position at eight o'clock in the evening. For a 
later hour turn it a little in the direction of the arrows. 
For example, in July, at ten o'clock, we hold it so as to 
have August at the top. The Roman numerals on top 

give the sidereal time 
without the trouble of 
calculating it. 

First find TJrsa Ma- 
jor, the Great Bear, 
generally called the Dip- 
per, an implement which 
the constellation resem- 
bles much more than it 
does a bear. This you 
can always do except perhaps in autumn when, if you 
are far south, it may be more or less below the northern 
horizon. Notice the pair of stars forming the outside 



♦ * 



Fig. 50. — Ursa Major, or The Dipper. 



308 



THE FIXED STARS 



^ STAR|^ 



Fig. 51. — Ursa Minor. 



of the bowl of the dipper. They are called the Pointers, 
because they point toward the pole star, as shown by the 

dotted line. This is the cen- 
tral star of the map. It is 
called Polaris. 

The pole star belongs to 

the constellation Ursa Minor, 

the Lesser Bear ; the rest of 

the constellation you will 

see by following a curved 

line of stars from the pole toward X\T hours. You will 

thus fall on another star as bright as Polaris but a little 

redder in colour. This is Beta Ursae Minoris. 

If you cannot see the pointers you will still easily find 
Polaris if you know the exact north, because it is nearly 
midway between the zenith and the northern horizon — 
nearer the latter, however, 
the farther south we are. It 
can be easily distinguished 
from its neighbour. Beta, 
by its whiter colour, Beta 
being slightly red or dingy 
in comparison. 

On the opposite side of 
the pole, at the same dis- 
tance as Ursa Major, is 

Cassiopeia, the Lady in the Chair. The chair has a very 
crooked back but could be made comfortable by a cushion 
in the hollow. 

There are several other constellations in the region 




Fig. 52. — Cassiopeia. 



THE AUTUMNAL CONSTELLATIONS 309 

around the pole, but they have few bright stars and are 
of less interest than those we have mentioned. Among 
them is Draco, the Dragon, whose form coils itself up be- 
tween the Bears, and whose head is represented by a tri- 
angle of stars in XVIII hours, near the August zenith. 

The Autumnal Constellations 

The zenithal and southern constellations to be looked 
for will vary with the season. We begin with the posi- 
tion of the sphere at hours sidereal time, which occurs 
at ten o'clock in October, eight in November, and six in 
December. 

The equinoctial colure is first to be imagined. It 
passes from the pole upward near the westernmost bright 
star of Cassiopeia and can be traced south through the 
eastern side of the square of Pegasus. The latter easily 
recognised landmark of the sky is formed by four stars 
of the second or third magnitude. The square is fifteen 
degrees on a side. 

Northeast from the northeast corner of the square is 
the Great Nebula of Andromeda. It is plainly visible to 
the naked eye as a whitish, ill-defined patch of light, and 
is a fine object when seen in a telescope. 

The Milky Way now spans the heavens like a slightly 
inclined arch, resting on the east and west regions of the 
horizon, and having its keystone a little north of the 
zenith, in Cassiopeia. Tracing it from this constellation 
toward the east, we first have Perseus, which stands in 
the Milky Way itself. The brightest star in this con- 
stellation is Alpha Persei, of the second magnitude. 



310 THE FIXED STARS 

East of Alpha is a white mass Hke a httle cloud. With 
a small telescope, even with a good field glass, we see this 
mass to be a collection or cluster of small stars. It is the 
Great Cluster of Perseus and, in the figure of the con- 
stellation, forms the hilt of the hero's sword. 

In a sort of offshoot toward the south (or southeast 
as the constellation is now situated) lies a row of three 
stars. The middle and brightest of these is the wonder- 
ful variable star, Algol, whose changes will be described 
in a later chapter. It is also called Beta Persei. , 

Below Perseus, the first large constellation is Auriga, 
the Charioteer. It is marked by Capella, the Goat, a star 
of the first magnitude and one of the brightest now above 
the horizon — indeed, one of the four or five brightest in 
the sky. But it has no other striking stars. 

In the southeast are Aldebaran and the Pleiades, 
which will be described later. Meanwhile let us follow 
the course of the Milky Way from the zenith toward 
the west. 

The first collection of bright stars west of Cassiopeia 
is now Cygnus, the Swan, lying centrally in the Milky 
Way. Five stars are arranged somewhat in the form of 
a cross and mark the body, neck, and extended wings of 
the bird. The brightest of the group is Alpha Cygni, 
or Deneb, nearly, but not quite, of the first magnitude. 

Low and to the right of Cygnus, and a little outside of 
the Milky Way, is the constellation Lyra, the Harp, 
marked by the beautiful and very bright bluish star, 
Vega. It has no other star of greater magnitude than 
the third, but what it has will repay careful study. 



THE AUTUMNAL CONSTELLATIONS 311 



In the figure given here, notice the star to the left of 
Vega ; Epsilon Lyrae it is called. A keen eye will, on care- 
ful examination, see that this star is really composed of 
two, lying so close together that it is not easy to dis- 
tinguish them. With an opera glass this will more easily 
be accomplished. But the 
most curious fact is that if 
a telescope be pointed at 
the pair, each of the stars 
will be found to be double, 
so that Epsilon Lyras is 
really composed of four 
stars. 

Another star, about as 
near to Vega as Epsilon is, 
lies at one corner of a par- 
allelogram or elongated 
diamond, which stretches 

south of Vega., At the farther blunt corner of the dia- 
mond lies Beta Lyras, marked B in the figure, a remark- 
able variable star. To the left of it is Gamma. The law 
of variation will be described in a later chapter. 

To the right of Lyra, and In the Milky Way, lies 
Aquila, the Eagle. It will be described later. 

The other constellations low in the west will be de- 
scribed later. At present we shall pass rapidly over the 
constellations of the Zodiac. 

If the ecliptic were painted on the sky we should now 
see it rising to the north of the east point of the horizon, 
passing in the south to mid-sky, where it would cross the 




Fig. 53. — Lyra, the Harp. 



312 THE FIXED STARS 

equator at a small angle, and then, passing to the west, 
reach the western horizon twenty-three degrees south 
of west. At the time we suppose, Sagittarius, the Archer, 
is mostly below the western horizon. Capricornus, the 
Goat; Aquarius, the Water Bearer, and Pisces, the 
Fishes, fill up the space to the meridian. The stars of 
these constellations are mostly faint, few or none exceed- 
ing the third magnitude. 

Reaching the meridian, we see the square of Pegasus 
above the Zodiac, not far south of the zenith. East of 
it is the constellation Aries, the Ram. Three of its prin- 
cipal stars, of the second, third, and fourth magnitudes, 
form an obtuse triangle. The brightest is Alpha Arietis. 

Two thousand years ago this constellation marked the 
first sign of the zodiac, and the equinox was just below 
Alpha Arietis, as explained in speaking of the precession 
of the equinoxes. 

Southeast from the square of Pegasus is a widely 
extended constellation, Cetus, the Whale. Its two bright- 
est stars. Alpha and Beta, are of the second magnitude. 
The latter lies nearly below the southeast star of the 
square of Pegasus and is quite by itself. Alpha is some 
distance farther east. West of Alpha, and a little south, 
is a remarkable star, Mira Ceti, the wonderful star of 
Cetus, which is invisible to the naked eye except for a 
month or two in each year, when it attains the fourth, 
third, and often the second magnitude. 

A little west of south, quite low down, is Fomalhaut, 
nearly of the first magnitude, in the constellation Pisces 
Australis, the Southern Fish. 



THE WINTER CONSTELLATIONS 313 




Fig. M.— The Hyades. 



The Winter Constellations 

The next position of the stars we shall describe comes 
six hours after the preceding one ; that is at two o'clock 
A. M. in November and at 
eight o'clock P. M. in Feb- 
ruary. During this six- 
hour interval another sec- 
tion of the Milky Way has 
risen in the east and passed 
over toward the south. The 
Milky Way now passes 
nearly through the zenith, 
resting on the horizon near 
the north and south points. 

Near its course and east of the meridian we see the 
constellation Taurus, the Bull, of which the brightest star 

is Aldebaran, form- 
ing the eye of the 
bull in the mytho- 
logical figure. Alde- 
baran is easily rec- 
ognised by its red 
colour. It lies on the 
end of one branch of 
a V-shaped cluster 
called Hyades. No- 
tice the pretty pair 
of stars in the middle 
of one leg. 




Fig. 55. — The Pleiades, as seen with the 



814 



THE FIXED STARS 



Near bj is the best known cluster in the sky, the 
Pleiades, or "seven stars." Only six stars are made out 
by ordinary unaided vision, but to a good eye five others 



TAYCETA^* 



MAlAi^ 



PkElOME 



<^t 



ALCYONE. • 



eLECTRA||fi 



MIROPE 



Fig. 56. — Telescopic View of the Pleiades^ with Names of the Brighter Stars. 



are visible, making eleven in all. The term "seven stars" 
is therefore a misnomer ; as a reason for it, it was said in 
ancient times that the number was originally seven but 
that one faded away. This "lost Pleiad" is probably a 
myth, as we do not find stars fading away permanently. 



THE WINTER CONSTELLATIONS 315 

With a telescope we find the cluster to contain quite a 
number of yet smaller stars, as can be seen by the tele- 
scopic view which we give. 

The central and brightest star of the group is 
called Alcyone, and was supposed by Maedler to be the 
central star of the universe. But this notion is quite 
baseless. 

East of Taurus and near the zenith is Gemini, the 
Twins, marked by two stars nearly of the first magnitude, 
Castor and Pollux. The latter is the northernmost and a 
little the brighter of the two. 

The next zodiacal constellation is Cancer, the Crab, 
but it contains no conspicuous stars. Its most noticeable 
feature is Prcesepe, a cluster of stars, which are singly 
invisible to the naked eye, and look collectively like a 
small patch of light. The smallest telescope will show a 
dozen stars in the patch. 

Leo, the Lion, is also well up in the east. It may be 
recognised by Regulus, a star nearly of the first magni- 
tude, and a curved row of stars in the form of a sickle, 
of which Regulus is the handle. 

In the south we now have the most brilliant constella- 
tion in the heavens, the beautiful Orion. The three stars 
of the second magnitude in a row forming the belt of the 
warrior are familiar from childhood to all who watch the 
sky. Below them hangs another row of three stars, the 
upper one quite faint. The middle one of these has a 
hazy aspect, and is really not a star at all, but one of the 
most splendid objects in the sky, the Great Nebula of 
Orion. A mere spy-glass will show its character, but a 



316 



THE FIXED STARS 



large telescope is required to bring out the magnificence 
of its form. 

The corners of the constellation are marked by four 
stars. The brighter of the two uppermost, Alpha 

Orionis, or Betel- 
guese, is reddish in 
colour. At the oppo- 
site corner is Rigel, 
blue in colour and 
also of the first mag- 
nitude. The two up- 
per stars are in the 
shoulders of the fig- 
ure. Midway and 
above them a triangle 
of small stars forms 
the head. 

East of Orion is 
Canis Minor, the Lit- 
tle Dog, containing 
Procyon, of the first magnitude. Below it and south- 
east of Orion is another collection of bright stars forming 
the constellation Canis Major, the Great Dog, containing 
Sirius, the Dog Star, the brightest fixed star in the 
heavens. 

The Spring Constellations 

The third position of the sphere, sidereal time twelve 
hours, occurs in February at two A. M. ; in May at 
eight P. M. Lyra has now risen in the northeast and 
Capella is going downward in the northwest. The Milky 




Fig. 61. — Ono7i. 



THE SPRING CONSTELLATIONS 317 



Way may not be visible at all unless the air is very clear. 
It will then be seen skirting the northern and western 
horizon. Regulus has passed the meridian, and Orion 
and Canis Major have set, or are low down in the 
southwest. 

In mid-heaven, southeast of the zenith, is Arcturus, of 
a dingy yellow col- 
our, but one of the 
brightest first magni- 
tude stars. 

East of Arcturus 
(now below it) is 
Corona Borealis, the 
Northern Crown, a 
beautiful semicircle of 
stars, of which the 
brightest is of the 
second magnitude. 

Near the zenith is 
Coma Berenices, the Hair of Berenice, a collection of 
faint stars mostly of the fifth magnitude. East of south 
across the meridian from Leo is Virgo, the Virgin, con- 
spicuous only by Spica, a white star of nearly the first 
magnitude. Libra, the Balance, east and southeast of 
Virgo, has no conspicuous stars. 

The Summer Constellations 

The fourth position of the sphere, eighteen hours 
sidereal time, occurs in May at two A. M. ; in August at 
eight P. M. Capella has now set, Lyra is near the zenith. 




Fig. 58. — 77ie Northern Crown. 



818 



THE FIXED STARS 




Fig. 59. — Aquila. 



Cassiopeia is in the northeast, and the most splendid por- 
tion of the Milky Way is near the meridian. We have 
described all the constellations 
that lie near its course north of 
Lyra; let us now trace it to the 
south. 

One of the noticeable fea- 
tures of the Milky Way now to 
be seen is the great bifurcation, 
or separation into two branches. 
The split can be traced from 
Cygnus, where it begins, past 
Lyra and halfway to the south- 
ern horizon. Here we see Aquila, 
the Eagle, in the cleft, marked by Altair, of the first 
magnitude. It is in a line between two other stars of the 
third and fourth magnitudes. 

At this point the westernmost branch of the Milky 
Way diverges yet farther and 
seems to terminate, but if the air 
is clear we shall see that it recom- 
mences near the horizon. 

East of Aquila is a small but 
very pretty constellation of which 
the scientific name is Delphinus, 
the Dolphin, but which is popu- 
larly known as Job's Coffin. 

Between Lyra and the beautiful 
Corona, now some distance west of 
the zenith, lies the widely extended 




Fig. 60. — Delphimis, 
Dolphin. 



THE SUMMER CONSTELLATIONS 319 




Fig. 6h—27ie Great Cluster of Eercuks, photographed at the Lick Observatory/. 



sw 



THE FIXED STARS 



constellation, Hercules. Alpha, its brightest star, is below 
the second magnitude and may be known by its reddish 
colour and by a white star. Alpha Ophiuchi, a little 
farther east. The most remarkable object in this con- 
stellation is the Great Cluster of Hercules which, to the 
naked eye, is a very faint patch, but which a great tele- 
scope resolves into a universe of stars. 

Near the horizon, west of south, is the zodiacal con- 
stellation ScorpiuSy the Scorpion. Its western boundary 

is a curved row of stars 
forming the claws of the 
animal; east of them is 
Antares, or Alpha Scor- 
pii, reddish in colour, 
and nearly of the first 
magnitude. 

In the Milky Way, 
due south, and therefore 
east of Scorpius, is Sag- 
ittarius, the Archer, wdth 
quite a collection of 
stars of the second and 
third magnitudes. The bow and arrow of the archer are 
easily imagined. 

Next toward the east are Capricornus and Aquarius, 
already mentioned. The brightest star in the former 
has a companion so close to it that it is a sign of not bad 
eyesight to be able to distinguish it. 



. • V 

• • * 



Fig. 62. — Scorpius, the Scorpion. 



IV 

The Distances of the Stars 

The principles on which distances in the heavens are 
determined was set forth in our chapter explaining how 
the heavens are measured. For distances of the moon and 
nearer planets, we use, as a base line for measurement, the 
radius of the earth, or the line joining two points of ob- 
servation on its surface. But this is entirely too short to 
serve for measuring a distance so great as that even of the 
nearest star. For this purpose we take as a base line the 
whole diameter of the earth's orbit. As the earth moves 
from one side of the orbit to the other, the stars must seem 
to have a slight motion in the opposite direction. But this 
motion is found to be almost immeasurably small. It can 
be made out with sufficient precision only by comparing 
the stars among themselves in the following way : 

Let the little circle on the left of the following figure 
represent the orbit of the earth. Let S be the star, sup- 
posed to be near us, of which we wish to measure the dis- 
tance. Let the dotted lines almost parallel to each other 
show the direction of a star T many times farther away. 
When the earth is at one side of its orbit, say at P, we 
measure the small angle SPT, which seems to us to sepa- 
rate these two stars. When the earth goes to the opposite 
side, it is readily seen that the corresponding angle SQT 
will be greater. We again measure it. The difference 



S22 THE FIXED STARS 

between these two angles will furnish a basis for com- 
puting, by trigonometric methods, the distance of the 
nearest star when that of the farthest is known. Practi- 
cally we have to assume that the star T is at an infinite dis- 
tance, so that the dotted lines are parallel. Then the 
measured difference between the angles will enable us to 
calculate the angle subtended by the radius of the earth's 
orbit, as seen from the star S. This angle is what astrono- 



FiG. 63. — Measurement of the Parallax of a Star. 

mers habitually use in their computations, not the dis- 
tance of the star. It is called the Parallax of the star. 
If we wish to obtain the distance of the star, we have to 
divide the number 206,265 by the parallax of the star 
expressed as a fraction of a second. This will give its 
distance in terms of the radius of the earth's orbit as a 
unit of measure. One second is the angle subtended by 
an object one inch in diameter at a distance of 206,265 
inches, or more than three miles. * It is, of course, com- 
pletely invisible to the naked eye. 

It will be seen that this method of measurement implies 
tliat we know which of the two stars is the nearer ; in fact, 
that we know the farther star to be at an almost infinite 
distance. The question may be asked how this knowl- 
edge is obtained, and how a star is selected as being near 
to us. The most careful measures that can be made with 
the finest instruments show that the great mass of small 



THE DISTANCES OF THE STARS S2S 

telescopic stars do not have the slightest change in their 
relative positions, but remain as if fixed on the celestial 
sphere from year to year. Now and then, however, an 
exception is found. A very bright star is probably nearer 
to us than the fainter ones, and if a star shows any 
change in its position, the astronomer may proceed to 
measure and determine its parallax. 

So far as has yet been determined, the nearest star to 
us is Alpha Centauri, a star of nearly the first magnitude, 
in the southern hemisphere. The parallax of this star 
is 0.75". By the rule we have given, its distance will be 
nearly 275,000 times that of the sun. Such a distance 
transcends all our power of conception over and over 
again. A crude idea of it may be obtained by reflecting 
that light itself, the speed of which we have already 
described, would require more than four years to reach 
us from this star. We see the latter, not as it is now, but 
as it was more than four years ago. At such a distance 
not only does the earth's orbit itself vanish away to a 
point, but a ball as large as the whole bedy- of Neptune 
would be barely visible to the naked eye as the minutest 
possible point. 

The next star in the order of distance is supposed to be 
about one half as far again as Alpha Centauri, and there 
are some half dozen others, within three or four times its 
distance. In all, the parallaxes of about one hundred 
stars have been determined with more or less exactness; 
but even in these cases the parallax is sometimes so small 
that we cannot be sure it is real. It seems likely that only 
about fifty stars are within seven times the distance of 



SM THE FIXED STARS 

Alpha Centauri. The distance of the stars whose paral- 
laxes are too small to be measured is a matter of judg- 
ment rather than calculation. The probability seems to 
be that at least the brighter stars are scattered through 
space with some approach to uniformity. If this is the 
case, many of the fainter telescopic stars, perhaps the 
large majority of the smallest ones found on photo- 
graphs of the heavens, must be more than one thousand 
times the distance of Alpha Centauri. The light by 
which their presence is made known to us must have been 
on its way to our system during the whole period of 
human history. 



V 

The Motions of the Stars 

If I were asked what is the greatest fact that the intel- 
lect of man has ever brought to light I should say it was 
this: 

Through all human history, nay, so far as we can dis- 
cover, from the infancy of time, our solar system — sun, 
planets, and moons — has been flying through space 
toward the constellation Lyra with a speed of which we 
have no example on earth. To form a conception of this 
fact the reader has only to look at the beautiful Lyra and 
reflect that for every second that the clock tells off^, we 
are ten miles nearer to that constellation. Every day that 
we live we are nearer to it by almost, perhaps quite, a 
million of miles. For every sentence that we utter, for 
every step that we take in the streets we are miles nearer 
to this star. We approached it by tens of thousands of 
miles while the writer has been penning these lines, and 
the reader has been carried nearer by a thousand miles 
while perusing them. This has been going on through 
all human history, and we have reason to believe that it 
will remain true for our remotest posterity. One of the 
greatest problems of astronomy is, when and how did 
this journey begin and when and how will it end.^ Before 
this question our science stands dumb. The astronomer 
can tell no more about the beginning or the end of the 



S26 THE FIXED STARS 

journey than can the untutored child. He can only im- 
press upon the mind of his followers the magnitude of 
the problem. 

Nothing can give us a better conception of the enor- 
mous distance of the stars than the reflection that not- 
withstanding the rapid motion, carrying us unceasingly 
forward through all the ages that the human race has 
existed on earth, ordinary observation would fail to show 
any change in the appearance of the constellation toward 
which we are travelling. From what we know of the dis- 
tance of Vega we have reason to suppose that our solar 
system will not reach the region in which that star is now 
situated until the end of a period ranging somewhere 
between half a million and a million of years from the 
present time. 

It does not follow, however, that our posterity, if any 
such shall then live on the earth, will find Vega when they 
arrive at its present j)lace. It also is going on its own 
journey and is passing away from its present location 
almost as rapidly as we are approaching it. 

What is true of our sun and of Vega is true, so far as 
we know, of ever}^ star in the heavens. Each of these 
bodies is flying straight ahead through space Hke a ball 
shot out from a cannon, with a speed which in most cases 
is almost inconceivable. It would be a very slow moving 
star of which the velocity did not exceed that of a cannon 
shot. In the great majority of cases it ranges from five 
to thirty miles per second — frequently more than fifty 
miles. Indeed there are two stars, of which Arcturus is 
one, whose speed we have reason to believe approaches 



THE MOTIONS OF THE STARS 327 

two hundred miles a second. These motions of the stars 
are called their proper motions. 

We have described the proper motions as so many miles 
per second. But owing to the enormous distance of the 
stars, rapid as the proper motions are in reality, they 
seem slow indeed when Ave observe them. So slow are they 
that if Ptolemy should come to life after his sleep of near- 
ly eighteen hundred years, and be asked to compare the 
heavens as they are now with those of his time, he would 
not be able to see the slightest difference in the configura- 
tion of a single constellation. Even to the oldest Assyrian 
priests, the constellation Lyra and the star Vega looked 
exactly as they do to us to-day, notwithstanding the im- 
measurable distance by which we have approached them.y 

To resuscitate an inhabitant of the ancient world who 
would be able to perceive any change, we should have to 
go back four thousand years perhaps, to the time of Job, 
and we should have to take one of the swiftest moving 
stars in the heavens, Arcturus. Bringing Job to life and 
showing him the constellation Bootes, of which Arcturus 
is the brightest star, he would perceive the latter to have 
moved through about half of the distance in the accom- 
panying diagram between the stars marked "1" and "2." 

In considering these motions, the most natural thought 
to present itself is that the stars are describing vastly ex- 
tended orbits around some centre, as the planets are 
moving round the sun, and that the motions we see are 
simply the motions in these orbits. But the facts do not 
support this view. The most refined observations yet 
made do not show the slightest curvature in the path of 



THE FIXED STARS 

any star. Every one seems to be going straight ahead on 
its own account, never swerving to the right or left. It 
does not seem possible to admit the existence of bodies 
large and massive enough to controlsuch rapid motions. 
A body massive enough to attract Arcturus from its head- 




FiG. 64. — Arciurus and the Surroimding Stars in Constellation Bootes. 

long course would throw all that part of the universe in 
which we live into disorder. The problem where the 
rapidly moving stars came from and whither they are 
going is therefore for us insoluble. What makes the case 
yet more difficult is that different stars move in different 
directions, without any seeming order, so that one motion 
seems to have no connection with another, unless in a few 
very rare cases. 



VI 

Variable and Compound Stars 

As a general rule the starry heavens may be taken as 
a symbol of eternal unchangeability. The proverb- 
makers have told us in all time how everything on the 
earth is subject to alternation and decay, while the stars 
of heaven remain as we see them, age after age. But it 
is now known that, although this is true of the great 
majority of the stars, there are some exceptions. These 
are so little striking that they were never noticed by the 
ancient astronomers. 

The first person in history to observe a change in a star 
was one Daniel Fabritius, a diligent watcher of the 
heavens, who lived three centuries ago. 

In August, 1596, he noticed a star of the third magni- 
tude before unknown in the constellation Cetus, which 
soon faded away again, and disappeared from view in 
October. In subsequent years it was found to show 
itself at regular intervals of about eleven months. 

Two centuries elapsed before another case of the kind 
was known. Then it was found that the star Algol, in 
Perseus, faded away from the second to the fourth magni- 
tude for a few hours at intervals of a little less than three 
days. 

Early in the nineteenth century other stars were found 
to be subject to a more or less regular variation of their 



330 THE FIXED STARS 

light. As observers studied the heavens with greater care, 
more and more of such stars were found, until at the pres- 
ent time the list of them numbers four or five hundred, 
and is constantly increasing. Of these some vary in an 
irregular way, but a large majority go through a regu- 
lar period. 

The easiest of these objects to notice is Beta Lyra, 
which is marked B on the figure of that constellation 
already given. It can be seen at some hour of any clear 
evening, spring, summer, or autumn. If the reader as he 
takes his evening walk will, night after night, compare 
this star with the one nearest to it and nearly of the same 
magnitude, he will see that while on some evenings the two 
appear perfectly equal, on others Beta will be of a mag- 
nitude fainter than the other. Careful and continued 
watching will show that the change takes place in a 
period of about six days and a half. That is to say, if 
the two stars are equal on a certain evening, they will 
again appear equal at the end of six or seven days, and 
so on indefinitely. Midway between the two times of 
equality the variable one will be at its faintest. If the 
observer notes the magnitudes at this time with the 
greatest precision, a curious fact will be brought out. 
Every alternate minimum, as the phase of least light is 
called, is slightly fainter than that preceding or follow- 
ing. The actual period is therefore nearly thirteen days, 
during which time there are two maxima of equal bright- 
ness and two slightly diff'erent minima. 

It is now known that the variation of light in this case 
Is not really inherent in the star itself, but arises from 



VARIABLE AND COMPOUND STARS 331 

the fact that the star is a double one, composed of two 
stars revolving around each other, and so near together 
as almost to touch. As they revolve, each one in succes- 
sion wholly or partially hides the other. This fact is not 
brought out by the telescope, because the most powerful 
telescope that could be made would not show the two 
stars separately. It is the result of long and careful 
study of the spectrum of the star, which is found to be 
a double one, the lines in one of which alternately cover 
and recede from the lines of the other. 

In the extent of variation of its light the most remark- 
able of the more conspicuous variable stars is Omicron 
Ceti, already mentioned as seen by Fabritius. It is now 
found to go through a regular period in three hundred 
and thirty days. During about two weeks of this time it 
is at its brightest, and is then sometimes of the second 
magnitude and sometimes much fainter — occasionally 
only of the fifth. After each maximum it gradually 
fades away for a few weeks and disappears from view to 
the naked eye. But with a telescope it can be seen all the 
year round. 

The period of eleven months makes the maximum occur 
about a month earlier every year. During some years it 
will occur when the star is so near the sun that it cannot 
be easily observed. This will be the case during the years 
1903-'05. 

Algol, also called Beta Persei, being in northern dec- 
lination, can be seen in our latitudes at some time on 
almost every night of the year. In autumn and winter 
it is visible in the early evening. The peculiarity of its 



332 THE FIXED STARS 

variation is that it remains of the same brightness nearly- 
all the time, but fades away for a few hours at intervals 
of about two days and twenty-one hours. It is now 
known that this is due to the partial eclipse of the star 
by a dark body nearly as large as itself, revolving round 
it. It is true that this body has never been seen hj human 
eye and never will be. Its existence is made known by its 
causing the star to revolve in a small orbit. It is true 
that this motion of the bright star Is too small to be 
observed with the telescope, but it Is made certain by 
means of the spectroscope, which shows a change in the 
wave length of the light coming from the star. 

Different variable stars differ very widely in the extent 
of their variation. In most cases the latter is so slight 
that only an expert observer would notice it. Frequently 
it cannot be determined until after a long study by 
various observers whether a "suspected variable" is really 
such. 

These objects form a very Interesting subject of ob- 
servation for those who have at command little or no 
instrumental facilities. No telescope is needed unless the 
star is, at some of Its phases, iuAdsIble to the naked eye. 
The points to be noticed and recorded are the exact 
magnitude of the star from minute to minute or hour to 
hour, as it is going through its most rapid change, in 
order to learn at what moment Its brightness is greatest 
or least. 

What adds to the Interest of the astronomer In these 
objects Is the evidence now being gathered that many, 
perhaps most of the stars, are not single bodies, but more 



VARIABLE AND COMPOUND STARS 3BB 

or less complex systems of bodies having the widest di- 
versity in their construction. Double stars have been 
familiar to every observer of the heavens since the time 
of the great Herschel. But it is only in the time of our 
generation that the spectroscope has begun to make 
known to us pairs of stars revolving round each other, 
of which the components are so close together that the 
most powerful telescope can never separate them. The 
history of science offers no greater marvel than the dis- 
coveries of invisible planets moving round many of the 
stars which are now being made, and in which the Lick 
observatory has recently taken the lead. 

It now seems more or less probable that the changes of 
light in all stars having a regular and constant period is 
due to the revolution of large planets or other stars 
around them. Sometimes the variation is slight and is 
caused in the way we have described, by one body par- 
tially eclipsing the other as it passes across it. In this 
case there may be no real variation in the light ; the star 
eclipsed shines just as bright behind the eclipsing body 
as when it is not eclipsed. But it now seems that, if the 
darker body revolves in a very eccentric orbit, so as to be 
much nearer the bright body at some times than at others, 
its attraction produces such a change in the other as to 
greatly increase its light. Just how this effect is pro- 
duced it is as yet impossible to say. 



THE END. 



BJL'07 



LRBFeZJ 



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